Update tox requirement from ==4.15.* to ==4.21.*

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.
- [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@4.15.0...4.21.0)

---
updated-dependencies:
- dependency-name: tox
  dependency-type: direct:production
...

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

requirements-test.txt

@@ -2,4 +2,4 @@ coverage == 7.5.*
 orjson == 3.10.*
 pre-commit == 3.5.*
 python-decouple == 3.8
-tox == 4.15.*
+tox == 4.21.*