Redo: split pytest plugins into individual modules - #6384

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog and making sure that this pull request is milestoned and labelled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

I think @pllim and @bsipocz are already aware, but for selfish reasons I would eventually like to see the testing module split out into a separate package.

Splittig out the test was the plan with #5770 and the gsoc project idea, so definitely a 👍 from me.

Thanks for picking this up @drdavella !

Note that the flake8 failure is real:

astropy/tests/pytest_doctestplus.py:341:1: W391 blank line at end of file

@Cadair - you've never rebased it. We since removed np1.7, the coverage test failure seems to be passing in fact, and the third one might have been fixed by the removal of the bundled pytest version...

ah bundled pytest. I am going to go ahead with the self-reinforcing belief that that was what it was ;)

I don't think this PR needs to be considered WIP anymore.

Thanks @drdavella and @Cadair!