Add tox.ini to easily test multiple Python versions

[hugovk]

Makes it easier to test locally. Also use it in GitHub Actions, this means the tox.ini file itself is tested.

Some example commands:

# Run on all Python versions listed in the tox file
# Missing versions are skipped
tox

# Run all in parallel (faster)
tox -p auto

# Run on a single Python version
tox -e py311

# Run on multiple Python versions
tox -e py38,py311

I left Python 3.12 out for now, because we can't yet test against the betas (#330 (comment)) and if we include it, then plain tox will try and run it and fail. We can still test on demand with tox -e py312.

I also formatted tox.ini using https://github.com/tox-dev/tox-ini-fmt (we could add that to a pre-commit hook).

[hugovk]

Okay, another thing blocked until we can move to 3.12 beta :)

Using tox to just {envpython} --version gives TypeError: rmtree() got an unexpected keyword argument 'onexc' on all alphas and not beta 1:

https://github.com/hugovk/blurb_it/actions/runs/5154209675

[AlexWaygood]

Okay, another thing blocked until we can move to 3.12 beta :)

Using tox to just {envpython} --version gives TypeError: rmtree() got an unexpected keyword argument 'onexc' on all alphas and not beta 1:

Same issue that I had in python/bedevere#547 (comment)!

[hugovk]

Close/re-open in case it helps the CLA bot.

[hugovk]

Updated to call pytest to measure/report coverage of all module and test code.

Similar to cherry_picker, and more accurately reports the current situation:

Before

Name                 Stmts   Miss Branch BrPart  Cover
------------------------------------------------------
blurb_it/error.py        2      0      0      0   100%
blurb_it/util.py        36     15      2      0    55%
tests/__init__.py        0      0      0      0   100%
tests/test_util.py      73     44      6      0    39%
------------------------------------------------------
TOTAL                  111     59      8      0    45%

After

Name                     Stmts   Miss Branch BrPart  Cover
----------------------------------------------------------
blurb_it/__main__.py       134    134     35      0     0%
blurb_it/error.py            2      0      0      0   100%
blurb_it/middleware.py      13     13      4      0     0%
blurb_it/util.py            36     15      2      0    55%
tests/__init__.py            0      0      0      0   100%
tests/test_util.py          73     44      6      0    39%
----------------------------------------------------------
TOTAL                      258    206     47      0    18%

[AlexWaygood]

The disadvantage of putting this config in pytest.ini rather than .coveragerc is that running pytest locally will now run pytest under coverage -- there's now no way to "just run pytest without a coverage report". Whereas if the config was in .coveragerc, we could run pytest locally to run the tests, and coverage run -m pytest to run the tests under coverage.

The tests in this repo are (at the moment) extremely fast, however, so there's not really any reason to not run tests under coverage. So I'm okay with this!

[AlexWaygood]

I don't use tox much myself, but this LGTM! The coverage config changes are a big improvement -- they highlight that we currently have a lot of code in __main__.py that isn't being exercised by our tests at all.