build(deps-dev): update tox requirement

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

Updates `tox` 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:development
  dependency-group: python-packages
...

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

requirements.dev.txt

@@ -2,7 +2,7 @@ pytest==8.3.3
 mypy==1.11.2
 black==24.8.0
 ruff==0.6.5
-tox~=4.18.1
+tox~=4.20.0
 pre-commit~=3.8.0
 build>=0.10.0
 coverage>=7.2.7