Update tox requirement from <=3.18.1 to <3.20.1

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.
- [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@0.5...3.20.0)

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

requirements.txt

@@ -13,7 +13,7 @@ pyyaml>=5.3.1
 requests>=2.24.0
 requirements-parser>=0.2.0
 tomlkit>=0.7.0
-tox<=3.18.1
+tox<3.20.1
 tox-battery>=0.6.1
 tox-envlist>=0.1.0
 trove_classifiers>=2020.5.21