add py.typed marker for type checking

[facutuesca]

Currently, when using a generated package as a library in another Python package and trying to run mypy for type checking, there is an error related to not being able to find neither library stubs or the py.typed marker.

# test.py
from my_cookiecutter_generated_package import x

$ mypy test.py
a.py:1: error: Skipping analyzing "my_cookiecutter_generated_package": module is installed, but missing library stubs or py.typed marker  [import-untyped]
a.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

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This PR adds an empty py.typed file to signal mypy that the generated package does provide type information (see docs here)

cc @woodruffw

[woodruffw]

Thanks @facutuesca!