Fix Continuous Integration Testing: Migrate from Travis to GitHub Actions

[tech234a]

• Migrate from Travis to GitHub Actions.
• Eliminate Coveralls (couldn't get it to work, it can be re-added later if someone can get it working).
• Add Ruff linting (currently set to pass on error, skipped on Python 2.7 due to incompatibility). (Included because the GitHub documentation example included it.)
• Update tested Python versions to 2.7, 3.7, 3.8, 3.9, 3.10, 3.11 (Python 3.3, 3.4, 3.5, and 3.6 are unavailable on the current GitHub actions image).
• Update README links and badges.

[TheTechRobo]

I thought it was going to be moved to a self-hosted CI...?