Update tox requirement from <3.18,>=3.1 to >=3.1,<3.21
Update tox requirement from <3.18,>=3.1 to >=3.1,<3.21

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](https://github.com/tox-dev/tox/compare/3.1.0...3.20.0)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
django-oscar/django-oscar-stores,django-oscar/django-oscar-stores,django-oscar/django-oscar-stores