chore: Bump tox from 4.12.1 to 4.15.0

Bumps [tox](https://github.com/tox-dev/tox) from 4.12.1 to 4.15.0.
- [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@4.12.1...4.15.0)

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

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

requirements/test.txt

@@ -1,7 +1,7 @@
 -r base.txt
 
 # Multi-environment testing
-tox ==4.12.1
+tox ==4.15.0
 
 # Wonderful py.test for testing
 pytest >=3.6.0