Bump virtualenv from 20.1.0 to 20.23.1 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.1.0 to 20.23.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.1.0...20.23.1)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

.github/workflows/constraints.txt

@@ -2,4 +2,4 @@ pip==20.2.4
 nox==2020.8.22
 nox-poetry==0.5.0
 poetry==1.1.2
-virtualenv==20.1.0
+virtualenv==20.23.1