Bump tox from 3.23.0 to 3.23.1

Bumps [tox](https://github.com/tox-dev/tox) from 3.23.0 to 3.23.1.
- [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@3.23.0...3.23.1)

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

requirements/ci.txt

@@ -1,6 +1,6 @@
 codecov==2.1.11
 docutils==0.17.1
-tox==3.23.0
+tox==3.23.1
 tox-travis==0.12
 pygments==2.9.0
 aiohttp==3.7.4.post0