Bump tox from 3.19.0 to 3.20.0 (#2602)

Bumps [tox](https://github.com/tox-dev/tox) from 3.19.0 to 3.20.0.
- [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@3.19.0...3.20.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

dev-requirements.txt

@@ -11,7 +11,7 @@ SQLAlchemy==1.3.19
 python-memcached==1.59
 swagger-spec-validator==2.7.3
 therapist==2.1.0
-tox==3.19.0
+tox==3.20.0
 WebTest==2.0.35
 werkzeug==1.0.1
 wheel==0.35.1