GitHub actions

[weaverba137]

This PR

• Fixes Don't assume master is default branch #154
  • The cost of eliminating master is that all branches must be explicitly labeled as such. For example desiInstall desiutil branches/main. Exception: desiInstall foo trunk is an abbreviation for desiInstall foo branches/trunk.
  • This may require updates to e.g. integration tests that install (checkout/clone) from scratch into a clean environment. However, I think the desispec nightly integration test doesn't actually do that, it just updates existing checkouts, right?
• Also implements GitHub Actions for desiutil.
  • However, tests of desiutil against Astropy 2.0.x are abandoned because it is not worth finding a compatible version of healpy that compiles in the GitHub environment.

[sbailey]

This looks great. The GitHub Actions are a breath of fresh air. I tested desiInstall for a tag and a branch and both worked. Minor items for consideration but not blocking factors for merging:

• Consider shortening the names of the tests in .github/workflows/python-package.yml. When viewing the summary of tests in the github PR interface, the right side is getting cutoff (e.g. "CI / Check code style; allow failures. (ubuntu-latest, 3.8) (pull_request) Suc") even when I have the browser window maximally wide on my laptop, and the interface doesn't provide a horizontal slider. I presume that if the test failed it would be flagged obviously, but short names would help, e.g. something like "test", "coverage", "docs", "style".
• very minor: could "desiInstall (package) main" be a shortcut for "desiInstall install branches/main" similar to "desiInstall (package) trunk"?

[weaverba137]

@sbailey, good suggestions. I'll get those in before merging.

[weaverba137]

@sbailey, do you have other significant changes to desiutil planned for release 20.12? If not, I can tag desiutil after this merge.

[sbailey]

No other planned changes for desiutil for 20.12; please tag after merging. Thanks.