Add py.typed file

[patrick91]

When using pylance/pyright you get a missing stub file error, see:

And this makes all usages of types trigger an error:

In order to fix this we need to tell the typechecker that this library is typed, by adding the py.typed file :)

See: https://blog.whtsky.me/tech/2021/dont-forget-py.typed-for-your-typed-python-package/

@samwillis let me know if I should bump the version too!

[JuroOravec]

@patrick91 I spoke with Sam, and we agreed that I'd fork this project and maintain it.

Please head over to https://github.com/JuroOravec/python-inline-source-2. Or just install sourcetypes2 instead of sourcetypes. The fork already as the py.typed added.