Replace codecov with python-coverage-comment-action

[ewjoachim]

Fixes #1647

@carltongibson said: @ewjoachim Input always welcomed 😜 -- https://github.com/carltongibson/django-filter/issues/1647#issuecomment-2001921040

I hope this is the kind of input you had in mind :)

The GHA code is mainly taken from the action's readme doc.

I'll merge it on my fork to check that it runs as expected.

Thanks for the kind nudge!

[ewjoachim]

(converting to draft until I can confirm it works on my fork)

[ewjoachim]

All good on my fork:

• The coverage branch was created at https://github.com/ewjoachim/django-filter/tree/python-coverage-comment-action-data
• Internal PRs have the comment
• External PRs too

(of course, the badge on the readme fails because it actually points to your fork and not mine, but I double-checked the link)