Bump tox from 4.18.1 to 4.21.0 in /requirements

Bumps [tox](https://github.com/tox-dev/tox) from 4.18.1 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@4.18.1...4.21.0)

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

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

Author: dependabot[bot]

requirements/tox.txt

@@ -1,3 +1,3 @@
 -r ./coverage.txt
-tox==4.18.1
+tox==4.21.0
 tox-gh-actions==3.2.0