Refactor the main Makefile

[jtraglia]

This PR refactors the Makefile so it's easier to read and use.

• Running make or make help will list all of the rules.
• It's no longer necessary to run make install_test or . venv/bin/activate.
• Cloning the repo and running make test will automatically create the virtual environment.
• You can use make test to run a specific test and/or fork; no need to manually use pytest anymore.
• Combine all linter checks into make lint.

Some screenshots: