Use Github Actions for testing

[samj1912]

Appveyor and Travis have been very slow lately. Github actions on the other hand provides an easy and fast way to test on all of our supported platforms.

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

[samj1912]

@Nurdok Appveyor test times for PRs have gone upto 1.5hrs per PR. I think this is a much better and unified way for CI moving forward.

As a plus, it also allows testing on MacOS.

And since all the runs happen parallely, testing is way faster.

[Nurdok]

Just a note - do you want to remove the AppVeyor / Travis config files? Or in a followup MR?

[samj1912]

I wanted to move them in a separate PR. That way once this is merged, we can add these as required checks and remove Travis and appveyor as required checks. Once they are removed as required checks, we can then safely remove the files.