Move to github actions for windows, mac, linux (x86, x86_64)

[mattip]

Since travis-ci.com is not free (it burns through credits that can be renewed), it would be nice to use free services available via github actions. We could even use drone.io for arm64, leaving ppc and s390x as the only archtectures left running on travis.

[mattip]

@matthew-brett Note I have disabled travis until this is fixed, see this comment

[mattip]

It also seems that in the move from matthew-brett to multi-build we lost appveyor CI, so this is critical for testing the repo

[mattip]

I tried enabling appveyor, but it seems we need some kind of credits.

[mattip]

I got appveyor started, but now the runs get stuck when building the anaconda environment

[cclauss]

#429 is now green on AppVeyor Python 3.7 only but it is a start...