tox 3.13.2 isn't compatible with python 3.8

[ivandasch]

Tox version, that is set in setup.py, isn't compatible with python 3.8.

I suppose, that it should be upgraded to at least 3.15.2. It requires six 1.15.0, therefore it should be also updated

[imcdo]

@ivandasch with your new change can this be closed?