Bump tox from 3.14.6 to 3.15.2

Bumps [tox](https://github.com/tox-dev/tox) from 3.14.6 to 3.15.2.
- [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@3.14.6...3.15.2)

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

requirements_dev.txt

@@ -5,7 +5,7 @@ bumpversion==0.5.3
 wheel==0.34.2
 watchdog==0.10.2
 flake8==3.7.9
-tox==3.14.6
+tox==3.15.2
 coverage==5.0.4
 Sphinx==2.4.4
 twine==3.1.1