Include type information

[cj81499]

I like to type check my Python code with mypy, but this library doesn't provide any type hints.

See PEP 561 for more info.

I haven't contributed types to a library before, but I may give it a shot myself if you're open to contributions from others.

[wimglenn]

I don’t use mypy but I’m not against it if someone else wants to make it work. We do still support Python 2, though, because in several problem cases for advent of code pypy2 is still significantly faster than any Python 3.X

[cj81499]

Sure thing! I've actually already started on this locally and hope to push to my fork soon.

I've been using "inline" (Python 3) style hints (it's much easier to write them this way), but I'll have to revise it to use type stubs (or type comments) before merge (or, drop python 2 support, but it's totally reasonable to prefer to keep compatibility).

I'll probably open a (very early) draft PR so you can start to look if you'd like.