Bump tox from 3.21.3 to 3.22.0

Bumps [tox](https://github.com/tox-dev/tox) from 3.21.3 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@3.21.3...3.22.0)

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

Author: dependabot[bot]

requirements_dev.txt

@@ -4,6 +4,6 @@ flake8==3.8.4
 pip==21.0.1
 pytest==6.2.2
 Sphinx==3.4.3
-tox==3.21.3
+tox==3.22.0
 twine==3.3.0
 wheel==0.36.2