Bump tox from 3.14.0 to 3.23.1 (#761)

Bumps [tox](https://github.com/tox-dev/tox) from 3.14.0 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@3.14.0...3.23.1)

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

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

Author: dependabot[bot]

requirements_dev_optional.txt

@@ -10,7 +10,7 @@ azure-storage-blob==2.0.1 # pyup: ignore
 redis==3.5.3
 pymongo==3.11.4
 # optional test requirements
-tox==3.14.0
+tox==3.23.1
 coverage
 flake8==3.9.2
 pytest-cov==2.12.0