Bump tox from 3.21.4 to 3.22.0 (#606)

Bumps [tox](https://github.com/tox-dev/tox) from 3.21.4 to 3.22.0.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/master/docs/changelog.rst)
- [Commits](tox-dev/tox@3.21.4...3.22.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

requirements/testing.txt

@@ -11,5 +11,5 @@ pytest==6.2.2
 pytest-cov==2.11.1
 pytest-timeout==1.4.2
 setuptools==53.0.0
-tox==3.21.4
+tox==3.22.0
 mypy==0.800