Bump virtualenv from 20.12.0 to 20.13.0

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:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

setup.py

@@ -46,7 +46,7 @@
             "python-coveralls==2.9.3",
             "coverage==4.5.4",
             "mock==2.0.0",
-            "virtualenv==20.12.0",
+            "virtualenv==20.13.0",
             "flake8==4.0.1",
             "pip-tools==6.4.0",
         ],