CI with GitHub Actions

[jGaboardi]

Prerequisite for solving #51

This PR addresses #56:

• migrates CI from TravisCI to GitHub Actions;
• adds testing for macOS
• adds testing for Python 3.8
• migrates code coverage from coveralls to codecov; and
• performs maintenance & updates relating to above (see Docs To Do #54,etc.).

Lacking (non-blocking)

• docstring tests (see here)
• Windows testing (see here)
• Python 3.9 testing (see here)

[jGaboardi]

@pedrovma I'll go ahead and merge this tomorrow unless you have any objections.