Switch to GitHub Actions for build testing

[haydenm]

This PR switches from Travis CI to GitHub Actions for build testing. It also:

• Adds linting before testing
• Expands testing to additional Python versions and macOS
• Changes the code coverage analysis from coveralls to codecov

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@204c36e). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #53 +/- ## ========================================= Coverage ? 93.83% ========================================= Files ? 63 Lines ? 6114 Branches ? 0 ========================================= Hits ? 5737 Misses ? 377 Partials ? 0 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `93.83% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute#carryforward-flags-in-the-pull-request-comment) to find out more. Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute)

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