investigate a mypy plugin

[bollwyvl]

On #162 , went down a rabbit hole trying to add a mypy plugin, similar to the coverage one.

This ended up being fairly maddening, as there are a lot of crystalized opinions about .endswith((".py", ".pyi")). Working around these ended up hitting roadblocks in a number of different directions, and as most users will be using a mypyc-compiled version of mypy, they can't be worked around simply.

Some levers:

• the PYTHON_EXTENSIONS global is overrideable
• a plugin can intercept the options being handed down, and can overload transform_source

Some pitfalls:

• non-.py(i) files need some special settings to not be interpreted as __main__
• it's fairly easy to end up with duplicate source files as the MypyFile.module (also the dictionary key handed to the plugin) seems to be pretty finicky