Switch GHA to use py-cov-action / python-coverage-comment-action

[Carlton Gibson]

Codecov's action broke. https://github.com/codecov/codecov-action/blob/main/README.md

See e.g. https://github.com/carltongibson/django-filter/actions/runs/8235019910

We've been meaning to get off it for a while.

Switch to @ewjoachim's https://github.com/py-cov-action/python-coverage-comment-action

[Joachim Jablon]

I'll be delighted to help if you folks need anything :)

[Carlton Gibson]

@ewjoachim Input always welcomed 😜