Bump virtualenv from 20.12.0 to 20.13.0 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.12.0 to 20.13.0.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.12.0...20.13.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -2,4 +2,4 @@ pip==21.3.1
 nox==2021.10.1
 nox-poetry==0.9.0
 poetry==1.1.12
-virtualenv==20.12.0
+virtualenv==20.13.0