Adopt GitHub Actions and Run Linters in Mutliple OSes and Python Versions

[gkapfham]

There are a few ways in which we can improve the use of continuous integration
for this project. Here are some examples:

• Start using GitHub Actions along with Travis CI for all the builds. Since it
  is possible to run Windows and MacOS builds with GitHub Actions, then we
  should adopt GitHub actions and start builds on Windows, Linux, and MacOS.
• GitHub Actions supports matrix builds, which would make it easy for us to run
  the test suite on multiple versions of Python (and also multiple operating
  systems, as mentioned in the previous point).
• If possible, we should investigate a way to have all the linting steps run
  through Pipenv scripts. This would provide a single standardized way in which
  all testing and linting commands can be run locally and in CI.

Does anyone else have any other ways in which we can improve how we run this
tool in a continuous integration environment?