Update virtualenv requirement from <20.16.3 to <20.25.2

Updates the requirements on [virtualenv](https://github.com/pypa/virtualenv) to permit the latest version.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@1.2...20.25.1)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-type: direct:production
...

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

requirements.in

@@ -10,4 +10,4 @@ pytest-cov
 Sphinx<4.4  # otherwise importlib-metadata dependency bites with flake8
 tox
 twine
-virtualenv<20.16.3  # otherwise importlib-metadata dependency bites with flake8. this is a dependency of tox.
+virtualenv<20.25.2  # otherwise importlib-metadata dependency bites with flake8. this is a dependency of tox.