[Feature Request] Declare project as PEP 561 (package supports typing) compatible

[stefanvanburen]

Feature description:

Ideally, protovalidate-python would declare that it is typing-compatible to please type-checkers, by supporting PEP 561.

Problem it solves or use case:

Currently, running mypy with on a project importing protovalidate shows errors, as we don't explicitly advertise typing support:

$ mypy .
hello.py:7: error: Cannot find implementation or library stub for module named "protovalidate"  [import-not-found]
hello.py:7: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

Proposed implementation or solution:

We should just need to add an empty py.typed file at the top-level of the source tree, and make sure that makes it into the built distributions.

Contribution:

I'll open a PR shortly.

Examples or references:

Similar situation in structlog package: https://github.com/hynek/structlog/tree/e97b83935e143d796c1ade553a609511239d5c36/src/structlog

Additional context:

N/A