Bump virtualenv from 20.4.2 to 20.4.3 in /.github/workflows (#42)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.4.2 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@20.4.2...20.4.3)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/constraints.txt

@@ -2,4 +2,4 @@ pip==21.0.1
 nox==2020.12.31
 nox-poetry==0.8.4
 poetry==1.1.5
-virtualenv==20.4.2
+virtualenv==20.4.3