Use Github Actions for CI

[langston-barrett]

Travis is shutting down its free CI for open-source projects, so we should migrate this project to Github Actions.

[langston-barrett]

Some helpful links:

• https://docs.github.com/en/actions/guides/building-and-testing-python#publishing-to-package-registries
  • https://docs.github.com/en/actions/reference/encrypted-secrets
• https://github.com/marketplace/actions/black-code-formatter
• https://github.com/actions/starter-workflows/blob/main/ci/python-package.yml