Build(deps): Bump tox from 3.20.0 to 3.20.1

Bumps [tox](https://github.com/tox-dev/tox) from 3.20.0 to 3.20.1.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/3.20.1/docs/changelog.rst)
- [Commits](tox-dev/tox@3.20.0...3.20.1)

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

Author: dependabot-preview[bot]

requirements/ci-wheel.txt

@@ -3,7 +3,7 @@
 
 coverage==5.3
 pytest-cov==2.10.1
-tox==3.20.0
+tox==3.20.1
 
 # Using PEP 508 env markers to control dependency on runtimes:
 codecov==2.1.9