Bump tox from 3.8.6 to 3.9.0 (#348)

Bumps [tox](https://github.com/tox-dev/tox) from 3.8.6 to 3.9.0.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/master/docs/changelog.rst)
- [Commits](https://github.com/tox-dev/tox/compare/3.8.6...3.9.0)

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

requirements/tox.txt

@@ -1 +1 @@
-tox==3.8.6
+tox==3.9.0