Bump tox from 3.21.2 to 3.21.3 in /requirements (#789)

Bumps [tox](https://github.com/tox-dev/tox) from 3.21.2 to 3.21.3.
- [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@3.21.2...3.21.3)

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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==3.21.2
+tox==3.21.3
 tox-gh-actions==2.4.0