CI: test bleeding edge Python

[andyfaff]

This PR proposes to add a CI test (using github actions) for bleeding edge releases of Python3.9. The purpose of this is so we don't get surprised when it gets released.
There is currently one test failure related to math.factorial deprecating the use of floats.
This is going to be a bit more unstable than released versions of Python.

[andyfaff]

Just added a commit that makes the CI run when:

• there's a commit to a PR
• there's a commit to master
• at midnight every day

This CI entry tests against the nightly build of Python 3.9

[tylerjereddy]

This is going to be a bit more unstable than released versions of Python.

Do we want this to show up as a genuine failure in the CI for every PR as opposed to i.e., an "allowed failure" of some sort that interested parties can investigate periodically?

[andyfaff]

I'd like to be able to do that, but I don't know how GitHub actions does an allowed failure.

[tylerjereddy]

This might be good enough: https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontinue-on-error

It may show as "always green" instead of i.e., "yellow," but maybe good enough for now..

[rgommers]

I don't know if "allowed failure" is useful. We will probably just ignore it then. Python is easy to build, so if one wants to just check the status occasionally, may as well do that locally.

We're past the 3.9 alpha stage now (beta 1 is planned for May 18th), so I'd be fine with trying this as a normal CI job. If it's too disruptive, let's just get rid of it completely.

[larsoner]

+1 for adding it as a standard test (not knownfail-like)

[tylerjereddy]

Ok, two core devs want Python 3.9 bleeding edge in as a regular CI test so I'll merge it, thanks.

I'm not sure we need the nightly test of the nightly builds anymore then, but maybe useful to try the cron feature out for a bit.

Also not sure we want it running after merge to master--I thought Ralf or someone on team advocated for deactivating that on Azure merges, which just skip on master merges I think.

Could also think about using OpenBLAS here, but if ATLAS is doing the trick for now maybe that's ok.

[tylerjereddy]

the PR one might be sufficient?

[tylerjereddy]

same here?