remove usage of esnext Set.union in case people are using older node versions

[DetachHead]

see #1106

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
-   /tmp/mypy_primer/projects/core/homeassistant/components/tplink/entity.py:111:16 - warning: Type of parameter "device" is unknown (reportUnknownLambdaType)
- 70619 errors, 348226 warnings, 0 notes
+ 70619 errors, 348225 warnings, 0 notes

spack (https://github.com/spack/spack)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/noversion-bundle/package.py:17:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/noversion-bundle/package.py:17:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/openblas/package.py:27:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/openblas/package.py:27:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/openmpi/package.py:14:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/openmpi/package.py:14:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:21:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:21:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:22:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:22:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:23:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:23:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:24:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:24:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:25:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:25:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:27:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:27:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:28:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:28:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:29:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:29:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:31:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test/package.py:31:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-2/package.py:19:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-2/package.py:19:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-2/package.py:20:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-2/package.py:20:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-3/package.py:18:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-3/package.py:18:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-3/package.py:19:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/optional-dep-test-3/package.py:19:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/parent-foo/package.py:20:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
+   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/parent-foo/package.py:20:5 - warning: Result of call expression is of type "((pkg: type[PackageBase]) -> None)" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)
-   /tmp/mypy_primer/projects/spack/var/spack/repos/builtin.mock/packages/parent-foo-bar/package.py:21:5 - warning: Result of call expression is of type "(pkg: type[PackageBase]) -> None" and is not used; assign to variable "_" if this is intentional (reportUnusedCallResult)

... (truncated 119 lines) ...

arviz (https://github.com/arviz-devs/arviz)