Combine ci workflows with tox

[jenhagg]

Purpose

I started this earlier and thought it might be useful to finish. The goal is to move the workflow to the tox.ini file rather than the github .yml so both local and ci usages will be able to use it. As part of doing that, I added isort since it's described in the wiki but we haven't been using it regularly, and this should make it easy.

What it does

First define tox.ini which allows either checking formatting (and running tests) with no side effects, or actually formatting the code locally before committing changes. Then combine these into the github workflow and delete the obsolete .yml. The pytest and formatting steps show up as separate checkmarks due to the strategy block in the .yml. This failed ci as expected, so I ran tox -e format which calls isort (in a way that is compatible with black), and is responsible for most of the changes.

To use it locally, running tox will default to running the pytest-local and format environments, which will in turn run the matching commands. Specifically, it will match the lines starting with pytest and local, as well as the formatting ones. Environments (called "testenvs" in tox language) can be invoked specifically, e.g. tox -e checkformatting would validate formatting and output a diff if it fails.

Note, this is meant to be totally optional, and running any of these as we do now will still work, the only change is that we would use tox in our ci pipeline.

Time to review

15 min - this isn't super important, I just thought I'd put this out if people are interested.

[rouille]

It works well. Thanks