Bump virtualenv from 20.3.0 to 20.3.1 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.3.0 to 20.3.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.3.0...20.3.1)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/constraints.txt

@@ -2,4 +2,4 @@ pip==20.3.3
 nox==2020.12.31
 nox-poetry==0.7.0
 poetry==1.0.10
-virtualenv==20.3.0
+virtualenv==20.3.1