Bump tox from 4.18.1 to 4.20.0

Bumps [tox](https://github.com/tox-dev/tox) from 4.18.1 to 4.20.0.
- [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...4.20.0)

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

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

pyproject.toml

@@ -55,7 +55,7 @@ dev = [
     "pre-commit == 3.8.0 ; python_version >= '3.9'",
     "pytest == 8.3.3",
     "pytest-cov == 5.0.0",
-    "tox == 4.18.1",
+    "tox == 4.20.0",
 ]
 # Docs are always built on Py3.12; see RTD and tox config files.
 docs = [