Bump virtualenv from 20.26.6 to 20.28.1 (#866)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.26.6 to 20.28.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.26.6...20.28.1)

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

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

requirements.txt

@@ -46,4 +46,4 @@ sphinxcontrib-serializinghtml==2.0.0
 sphinxcontrib-spelling==8.0.0
 typing_extensions==4.12.2
 urllib3==2.2.3
-virtualenv==20.26.6
+virtualenv==20.28.1