Add tox file to test on all currently supported Python versions

[jdevera]

Also fixed some tests.

This could be a first step towards integrating with TravisCI, perhaps.

[tedder]

Thanks for this- here's what I've done:

• included the test fixes in my modernization commit, though implemented both of them slightly differently: headers, regex
• used Github Workflows for the automation; note the version list I'm actually testing against:
• started slowly changing the classifiers in a branch, this will be my final commit before going from 1.0 to 1.1.0.

I still need to update the version classifiers. It's probably too difficult to leave your commit credit for that, but I do hope you submit more PRs and help out! I'm trying to increase test coverage and update things around here.