add py.typed marker indicating provision of inlined types

[acartine]

when enabling pylance/pyright typing i encountered some errors related to flask-pydantic. when i reached out to the maintainer

https://github.com/microsoft/pyright/discussions/4733#discussioncomment-5207328

he provided this feedback.

The flask_pydantic package appears to have inline type annotations within its own code. However, it is not marked as "py.typed" (i.e. it doesn't have a "py.typed" marker file indicating that it has inlined types). You may want to reach out to the maintainers of the library to encourage them to add this marker file. For more details, refer to this documentation.

i'm happy to give this a shot with a PR but wanted to have some discussion about it first. Thanks in advance!