Too late for Valentines Day

Revise for 6.0.0 Mathics3 API and current Mathics3 builtin standards described in Guidelines for Writing Documentation.

Thie package has undergone a major overhaul. Modules have been split out along into logical groups following the documentation structure.

We have gradually been rolling in more Python type annotations and have been using current Python practices. Tools such as using isort, black and flake8 are used as well.

Evaluation methods of built-in functions start eval_ not apply_.

There is more refactoring more to do here, Upgrade to NetworkX is also desirable.