Adds PEP 561 Compatibility

[stumpylog]

Fixes #2

This adds a py.typed file to the package data, and runs mypy against the code base to almost make it clean. 1 typing error remains which I'm unusre how to fix, but it's quite minor. The typing changes are easily to revert if desired.

[coveralls]

coverage: 93.284% (+0.1%) from 93.182% when pulling d84b540 on stumpylog:main into 822a1cd on un33k:main.

[un33k]

Thank you.