TST: migration to pytest-mpl (Github actions)

[neutrinoceros]

Now that #4122 is in, there are a couple more steps needed, the first two of which can only be done by repo owners (@matthewturk , @Xarthisius)

• Build and Test / answer tests: py3.9 (which doesn't do anything any more) should not be required by auto-merge
• Build and Test / Image tests replaces it, it should be required instead
• Build and Test / answer tests: py3.9 can be removed completely (this can be done in a PR)
• Document image updating process -> DOC: document pytest-mpl update workflow #4418
• drop answer test submodule

[neutrinoceros]

I got (at least) one thing wrong when I wrote this: Build and Test / answer tests: py3.9 isn't doing nothing, but it should still retire it (pytest-mpl tests are already actively replacing it). We could limit our action to making it not required for now, and drop it sometimes in the future when it becomes unmaintainable.