⬆️ Update tox requirement

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@4.18.1...4.21.0)

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

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

pyproject.toml

@@ -68,7 +68,7 @@ dev = [
     "pre-commit~=3.8.0",
     "mypy~=1.11.2",
     "ruff~=0.6.5",
-    "tox>=4.18.1,<4.21.0"
+    "tox>=4.18.1,<4.22.0"
 ]
 
 [project.scripts]