Add tox config

[medecau]

This pull request adds a tox config file to make new contributions easier.
Listed are environments for Python 3.5, 3.6, 3.7, 3.8, 3.9, and pypy3.

tox aims to automate and standardize testing in Python.

To use:

$ pip install -U tox
$ tox
# for parallel testing
$ tox -p 4

More information at:
https://tox.readthedocs.io/

[eriknw]

Cool, thanks @medecau. Sorry for the delay. I'm striving to be more responsive.