Consider github actions for CI instead of travis, appveyor, etc

[ceball]

I recently tried out github actions for pyctdev because I couldn't log into our pyviz-dev GH account to control appveyor. It was very easy and quick to get builds for windows, linux, and macos. And the resulting config appears simpler than what you end up with for other systems (i.e. multiple config files for the different platforms, plus multiple repeated parts across (and within!) projects). Also, the builds start and run very quickly (of course that could change in future). Plus the resulting build info is better integrated with github.

Anyone can define steps/workflows, which can then be re-used ("imported and run") by anyone else. We should be able to create a couple of actions for use across all our projects (where the actions are not already provided by github or someone else). Additionally, organizations can provide default workflows, so some projects might not even need their own CI config.

So, seems like github actions would probably both simplify and improve our CI - something to bear in mind given we are unlikely to have time to switch everything over at the moment.

I suppose the downsides are (a) not much experience of github actions yet and (b) increased reliance on github (although for (b) I would keep arguing for steps that can be run locally as well as on CI wherever practical, so they can always be migrated to the next system...).

[jbednar]

Sounds good. I also strongly advocate for (b).

[philippjfr]

Bokeh just migrated their CI setup over to github actions. Their setup is very different but it might be worth looking at it anyway to see if there's anything we can crib from them.