Add py.typed for type hints

[UFO-101]

Description

I think that all libraries are supposed to include a py.typed file for types to be accessible to external users. See: https://blog.whtsky.me/tech/2021/dont-forget-py.typed-for-your-typed-python-package/.

However, I haven't researched this thoroughly, so someone should probably look into this a bit more.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have not rewritten tests relating to key interfaces which would affect backward compatibility

[UFO-101]

Bump @bryce13950

[bryce13950]

Sorry for the delay on getting this in, and thanks for bumping me. I updated it to the most recent version of TransformerLens, and that seems to have caused some issues. I am thinking of rolling this into 3.0. The issues that came up, I believe will already be fixed in dev-3.x. If you are fine with that, I will update the merge point to dev-3.x and work this into 3.0