Bump virtualenv from 20.0.31 to 20.0.33 in /.github/workflows (#587)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.0.31 to 20.0.33.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.0.31...20.0.33)

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{{cookiecutter.project_name}}/.github/workflows/constraints.txt

@@ -2,4 +2,4 @@ pip==20.2.3
 nox==2020.8.22
 nox-poetry==0.5.0
 poetry==1.0.10
-virtualenv==20.0.31
+virtualenv==20.0.33