Add py.typed file (PEP 561)

[martijnthe]

Summary

Mypy only considers the type annotations of packages that contain the py.typed marker file, even if they are fully type-annotated (like pybreaker). See https://peps.python.org/pep-0561/

This PR adds said file.

I had to move pybreaker into a module instead of a single .py file, because there is no support for py.typed files for single .py file packages yet (see https://github.com/python/typing/issues/1333).

This solves https://github.com/danielfm/pybreaker/issues/84

Test Plan

• Ran python setup.py test to ensure the tests still pass.
• Ran python -m build to build the package.
• Observe the output logs to see that py.typed was included.
• Install the tarball into a project where I have mypy set up. Run mypy and observe mypy now no longer fails with ... becomes "Any" due to an unfollowed import [error/no-any-unimported] for imports from pybreaker.

[danielfm]

Hey, thanks for the contribution!