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] <support@github.com>

pyproject.toml

@@ -63,7 +63,7 @@ bump2version = "1.0.1"
 wheel = "0.44.0"
 watchdog = "4.0.2"
 flake8 = "5.0.4"
-tox = "4.20.0"
+tox = "4.21.0"
 coverage = "7.6.1"
 twine = "5.1.1"
 pytest = "8.3.3"