Bump tox from 3.19.0 to 3.20.0

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[bot] <[email protected]>

Author: dependabot[bot]

requirements_dev.txt

@@ -5,6 +5,6 @@ mock==3.0.5
 pip==20.2.2
 pytest==6.0.1
 Sphinx==3.2.1
-tox==3.19.0
+tox==3.20.0
 twine==1.15.0
 wheel==0.35.1