Migrating to GH actions

[dstansby]

Provide a general description of the issue or problem.

Since our ability to use Azure pipelines is dwindling, we've decided to re-create our CI on Github actions. This has already been started, but there is still plenty to do. Off the top of my head the list is below; please edit/add items!

Tests

Write a GitHub workflow to replace the azure pipelines tox workflow, currently here but ultimately should be in an openastronomy repo.

• Install list of libraries
• Install and execute tox
• Install tox-pypi-filter
• Codecov
• Test inside a conda environment
• [N/A] Skip CI doesn't skip required builds (if even possible) Don't think this is possible unless we use an alternative command such as [sunpy skip].
• (optional) xvfb
• (optional) Run inside docker container [32bit and python betas]

Release

• Write reusable action for building sdist & universal wheels (https://github.com/OpenAstronomy/action-build-sdist-and-wheel)
• Add wheel builds using cibuildwheel
• Write a reusable workflow which combines sdist, cibw and pypi upload (https://github.com/OpenAstronomy/github-actions-workflows)
• (optional) add support for nightly builds with dev version support

Sunpy specific things:

• Cron jobs
• Matrix notifications (https://github.com/KrazyKirby99999/matrix-commit)
• Break the jobs up into stages like we had on Azure to fail early.
• Build wheels and test them on branches (not PRs)
• Implement release on successful test run on tag

[Cadair]

I had a call with Azure support today, and they were very helpful and understanding but ultimately unable to clarify what exactly was causing their automated abuse detection to shut us down, so I think we are left with few other options than to abandon ship.