Bump tox from 2.3.1 to 3.14.1

Bumps [tox](https://github.com/tox-dev/tox) from 2.3.1 to 3.14.1.
- [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@2.3.1...3.14.1)

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

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -3,7 +3,7 @@ bumpversion==0.5.3
 wheel==0.29.0
 watchdog==0.8.3
 flake8==2.6.2
-tox==2.3.1
+tox==3.14.1
 coverage==4.1
 Sphinx>=1.4,<1.5
 cryptography==2.3.1