Add py.typed to package data

[kylebarron]

I'm trying to understand the mypy ecosystem better. In the original PEP 561, it references a simple script to check if your package correctly exposes types, and that alleges that rio-tiler isn't exporting types because py.typed isn't included in the distributed output. In the Mypy documentation, it instructs you to include py.typed manually in your package data output.

That said, in an example script that just imports rio_tiler, running mypy test.py doesn't complain about being unable to follow the rio-tiler import, so I can't tell if this is necessary.