WIP: Refactor `prerequisites` module and tests

[langfield]

This commit bundles the former is_prerequisites_tag(), strip_pre_prefix(), and extract_prerequisites_tags() into a single extract_prerequisite_tags() function. It also adds type hints, lints all source files, and generally pushes the repository into a PEP-compliant direction.

• Add default py3 .gitignore.
• Rename tests/main.py -> demo.py.
• Add pylintrc.
• Add setup.py.
• Normalize package imports.
• Make tests/ a python package.
• Refactor demo logic.
• Make printing in demo script a bit prettier.

[pwintz]

Thanks! This handles some of the repository organization that I wanted to do but hadn't gotten around to yet.