MAINT: pysat 3.0 release, Github Actions

[jklenzing]

• Updates the CI environment to use Github Actions rather than Travis CI
• Installs pysat 3.0 directly from pypi
• Uses flake8 rather than pytest-flake8 for immediate feedback if a style error is present

[jklenzing]

Verified that flake8 migration works as expected. Tagging all of you for discussion at the meeting this week. This pull should be a clean replacement to #48 and #45.

[aburrell]

Should we just do the flake8 test on one python version?

[jklenzing]

My main thought here is that it doesn't matter. Flake8 is now decoupled from the tests. It is currently set up so that if flake8 fails, the tests stop. The upside to this is that you don't have to wait for an hour to see you have a style error.

Long-term, I imagine adding a second workflow that only runs once for the documentation checks. We could potentially move the flake8 check there to fully decouple from the multi-environment tests.

[rstoneback]

Thanks @jklenzing. Approving my part, not getting ahead of @aburrell's comments.

[jklenzing]

Thanks @rstoneback. Going to leave this here for discussion on Friday