Add new github actions.

[samuelgarcia]

@JuliaSprenger @apdavison @mdenker
This GH actions will replace travis one.
It run on several os/python/numpy.

[samuelgarcia]

If tests pass I will remove Travis

[samuelgarcia]

This is now 45 jobs (3 * 3 * 5) for the core.
Maybe we could remove some numpy version.
@apdavison : do you remember what was the rules for numpy version testing ?
Here I already change the list compared to the travis CI

[JuliaSprenger]

Looks good to me. Do you think it would make sense to run the io tests only once the core tests succeed or should be keep this independent?

[samuelgarcia]

Yes I was think something like this : run io and rawio only if core is passing (~3min).
I think we can serialize workflow, I will check.

[JuliaSprenger]

Ok, then I will merge this in the current state and we can add chaining later.

[apdavison]

For testing, we decided on NEP29 + 1 year, so we should be testing Python 3.7+ and NumPy 1.17+. I suggest we test all combinations for the core tests, but only the oldest and newest for io tests.

[samuelgarcia]

thanks for merging.

Andrew, for cortest you suggest all numpy version from 1.17 to 1.22 ?
It is a lot no ?
If we do this maybe we could remove osx testing ?
otherwise it will be 66 jobs!! between 1min to 3min each!
numpy is not so instable regarding API and behavior even if from time to time we have a bad surprise.
I would test oldest/newest and problematic ones, no ?

For iotest I don't think we have to go for a matrix f os/python/numpy.
It already take 13min with chaching file and installation.

[apdavison]

In this PR you're already testing 5 NumPy versions, so I'm suggesting only one more. I would agree about removing osx testing, and maybe we can also run a smaller matrix on Windows.

[samuelgarcia]

Lets discuss in this PR #1096