Build(deps): Bump pip from 21.0.1 to 21.1 in /.github/workflows (#351)

Bumps [pip](https://github.com/pypa/pip) from 21.0.1 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@21.0.1...21.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
-pip==21.0.1
+pip==21.1
 nox==2020.12.31
 poetry==1.1.6
 virtualenv==20.4.3