ENH: add tox.ini to generize test setup

[bsipocz]

This should help local testing, etc.

To close #141

It also addresses (potentially), and close #139

TODO (but opening the PR now to see whether this indeed works on GHA not just locally):

• rethink the matrix and think about version combinations that make sense
• add dev version testing to the matrix
• use tox for the other CI configs, including building the HTML pages

[bsipocz]

hmm, tox -e test works locally, not sure what goes wrong with it (though in other projects I don't have multidimensional python version, but add that to the environment).

[bsipocz]

@rossbar @melissawm - Far from perfect, and finished, but maybe it's better to include more things (e.g. see the checkboxes above) into tox incrementally.

[bsipocz]

Let's get this in, we can always improve incrementally.

[melissawm]

Thanks, @bsipocz !!