Bump virtualenv from 20.2.0 to 20.2.1 in /.github/workflows (#174)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.2.0 to 20.2.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.2.0...20.2.1)

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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==20.2.4
 nox==2020.8.22
 poetry==1.1.2
-virtualenv==20.2.0
+virtualenv==20.2.1