build(deps-dev): bump tox from 4.20.0 to 4.21.0

Bumps [tox](https://github.com/tox-dev/tox) from 4.20.0 to 4.21.0.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/main/docs/changelog.rst)
- [Commits](tox-dev/tox@4.20.0...4.21.0)

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

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

requirements-dev.txt

@@ -1,4 +1,4 @@
 -r requirements.txt
 coverage
 pre-commit==3.5.0
-tox==4.20.0
+tox==4.21.0