Bump tox from 4.18.0 to 4.21.0 (#1213)

Bumps [tox](https://github.com/tox-dev/tox) from 4.18.0 to 4.21.0.
- [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@4.18.0...4.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pyproject.toml

@@ -46,7 +46,7 @@ dev = [
  # Setup.
  "pre-commit==3.8.0 ; python_version>='3.9'",
  "tox==3.27.1 ; python_version<'3.8'",
- "tox==4.18.0 ; python_version>='3.8'",
+ "tox==4.21.0 ; python_version>='3.8'",
  # Testing framework.
  "pytest==6.1.2 ; python_version<'3.8'",
  "pytest==8.3.2 ; python_version>='3.8'",