Add `py.typed` to support type checking

[Trenton H]

I think this should be as easy as adding an empty file named py.typed and maybe adding to package_data in setup.py. This would let users take advantage of the great typing the library has with tools like mypy. Right now, mypy gives lines like:

error: Skipping analyzing "imap_tools": module is installed, but missing library stubs or py.typed marker  [import]

[Vladimir Kaukin]

#207