Bump virtualenv from 20.0.29 to 20.0.31 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.0.29 to 20.0.31.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/master/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.0.29...20.0.31)

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

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
 pip==20.2
 nox==2020.5.24
 poetry==1.0.10
-virtualenv==20.0.29
+virtualenv==20.0.31