Updated python-tox to the latest version 3.1.2.
Changed packaging so that both tox/tox3 are available to the user.

Changelog:

• python-packaging >= 17.1 is now an install dependency. Depends on D3423.
• Add support for multiple PyPy versions using default factors. This allows you to use, for example, pypy27 knowing that the correct intepreter will be used by default.
• Add support to explicitly invoke interpreter directives for environments with long path lengths.

Full changelog for skipped versions available here.

Installed and ran several test suites from different git repositories with everything working as expected.
Tested with both Python2/Python3 versions launched with tox/tox3.