Add tox.ini to make local testing easy

[hugovk]

To use:

python -m pip install tox
tox               # to test all versions
tox -s            # --skip-missing-interpreters, if you don't have a version installed
tox -e py310      # to test a single version
tox -e py27,py310 # to test more than one
tox -p auto       # run in parallel

For example: