Bump tox from 3.21.3 to 3.21.4 in /requirements (#790)

Bumps [tox](https://github.com/tox-dev/tox) from 3.21.3 to 3.21.4.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/3.21.4/docs/changelog.rst)
- [Commits](tox-dev/tox@3.21.3...3.21.4)

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

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

Author: dependabot[bot]

requirements/tox.txt

@@ -1,3 +1,3 @@
 -r ./coverage.txt
-tox==3.21.3
+tox==3.21.4
 tox-gh-actions==2.4.0