Use GitHub Actions

[mdpiper]

This PR switches CI for the project from Travis CI to GitHub Actions. It includes workflows to build/test, format with black, and lint with flake8.

Notes:

• I formatted versioneer.py. We're going to remove it eventually, and doing so lets the the format job pass.
• There's an issue on Windows (Error importing bmipy-render output on Windows #10) that causes a test to fail, so I skipped it. Unskip it once the issue is resolved.