Remove codecov dependency; skip running CI on 3.12 for now

[AlexWaygood]

The codecov package is no longer installable, meaning that pip install dev-requirements.txt for this repo now always fails. It also doesn't seem to be necessary to install it in CI; we don't use it for anything as far as I can see, and the codecov action doesn't require it.

[codecov]

Codecov Report

Merging #547 (48ecae9) into main (953f6ff) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #547   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         2030      2030           
  Branches       231       231           
=========================================
  Hits          2030      2030           

Flag	Coverage Δ
Python_3.10.10	?
Python_3.10.11	100.00% <ø> (?)
Python_3.11.2	?
Python_3.11.3	100.00% <ø> (?)
Python_3.12.0-alpha.4	?
Python_3.8.16	100.00% <ø> (ø)
Python_3.9.16	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[AlexWaygood]

It looks like the tests are failing on 3.12, but in actual fact, we never get as far as running the tests! tox tries to use virtualenv to set up a virtual environment, but virtualenv crashes. Why? It tries to pass in an onexc argument to shutil.rmtree, because virtualenv sees that we're running on Python 3.12, and wants to avoid a DeprecationWarning (since the old onerror argument to rmtree is deprecated in Python 3.12). But we pin 3.12.0a4 following 5f61b88, meaning the onexc argument to shutil.rmtree doesn't exist yet (it was added in 3.12.0a7, in python/cpython@7f760c2).

Not sure what the best solution here is! The original reasons for pinning the 3.12 workflow to 3.12.0a4 in #542 still apply -- aiohttp and frozenlist still don't support being installed on Python 3.12.0a5 or higher.

Maybe we should just skip the 3.12 workflow altogether for now?

[hauntsaninja]

Yeah, I'd skip the 3.12 workflow for now