Add a py.typed file so that mypy will know to use your inline type annotations

[ted-marozzi]

Is your feature request related to a problem? Please describe.
The problem is I have to reimplement your typings that you already maintain with your inline types, this is extra work for no reason.

Describe the solution you'd like
Add a py.typed file to the dist bundle so that mypy will use your inline types.

Describe alternatives you've considered
Reimplementing the typings on our end. This in unnecessary and cost time + effort.

Additional context
Here is some additional info on py.typed

[edgarrmondragon]

I opened a PR to address this: #362