GitHub Actions

[smmaurer]

This PR moves the automated testing from Travis and AppVeyor to GitHub Actions. This lets us break up the tests into clearer and more modular chunks. GitHub is also running them much faster.

We'd like to gradually move all the UDST repos over to this model. Pandana is the first.

Tests running on GitHub Actions

On push:

• check code style
• run unit tests in a single generic environment (ubuntu-py38)

On PR:

• generate a coverage report
• run unit tests in a range of python versions and operating systems

Weekly:

• install Pandana from Pip and Conda in a range of environments, to catch dependency updates that break Pandana

Other changes in this PR

1. Updates a testing setting that skipped certain tests on Travis -- now skips them on any CI system.

2. Drops the MACOSX_DEPLOYMENT_TARGET environment variable from the installation script. This has been in there for years and occasionally causes problems on certain systems -- including the GitHub Actions macos-latest image. It doesn't seem necessary to set a specific value any more. (Tested in 10.15 Catalina, 11.0 Big Sur, and GitHub Actions macos-latest.)

There are a zillion commits here, because I was learning how GitHub Actions works, so I'll do a squash merge.

[coveralls]

Coverage increased (+0.2%) to 89.886% when pulling c866c0f68a72f51896d45d5448786a6f52825e04 on github-actions into 20af9a5b0854e528220774f1a816336098b15c57 on dev.