Add `py.typed` to expose typing information to clients

[kkirsche]

Per PEP 561 packages which distribute type information must have a py.typed file which is included in the distributed package. This is specifically discussed in the Packaging Type Information section if you'd like to review this in more detail.

This issue is to ask if you are open to this before submitting a merge request to add this so that type information is exposed to user's projects

Thanks!

[davidkpiano]

Yes I am, but we will be updating this package significantly, so it may be better to wait until then.

[leetrout]

@davidkpiano I know it's everyone's favorite question.... but do you know when that work might be happening? ☺️