Bump virtualenv from 20.0.28 to 20.4.3 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.0.28 to 20.4.3.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.0.28...20.4.3)

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

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
 pip==20.1.1
 nox==2020.5.24
 poetry==1.0.10
-virtualenv==20.0.28
+virtualenv==20.4.3