Bump virtualenv from 20.14.1 to 20.15.1

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.14.1 to 20.15.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.14.1...20.15.1)

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

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

setup.py

@@ -46,7 +46,7 @@
             "python-coveralls==2.9.3",
             "coverage==4.5.4",
             "mock==2.0.0",
-            "virtualenv==20.14.1",
+            "virtualenv==20.15.1",
             "flake8==4.0.1",
             "pip-tools==6.6.2",
         ],