Bump virtualenv from 20.26.2 to 20.28.1

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.26.2 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@20.26.2...20.28.1)

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

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