Fix pytest-mpl GitHub Pages config

[ConorMacBride]

Some pytest-mpl tests need to access http://matplotlib.github.io/pytest-mpl/2.0.x/test_succeeds_remote.png however a 404 is returned (after 301 redirecting to https://matplotlib.org/pytest-mpl/2.0.x/test_succeeds_remote.png). Could someone check that GitHub Pages is enabled in the repo settings? Maybe there has been a change at the organisation level? I can't remember if the custom domain was configured before.

[Cadair]

GH pages is configured, and the domain appears to be setup from the org level, so I have asked in the main mpl chat.

[tacaswell]

Ah, sorry, we missed this when we moved to self-hosting the docs (the main mpl docs were getting to big to deploy via gh pages!).

matplotlib/matplotlib.org#16 needs to be merged and we need to add a webhook to tell the new server to clone pytest-mpl when there is a push to gh-pages.

attn @QuLogic

[QuLogic]

I had this on my list, but the branch hasn't been updated in 4 years, so I thought you weren't using it.

[QuLogic]

This is online now.

[Cadair]

Thanks very much @QuLogic & @tacaswell 🚀