Automatically add `.pyi` files to manifest

[leandro-lucarella-frequenz]

When using the setuptools.grpc tools, the user needs to specify recursive-include py *.pyi in the MANIFEST.in file, otherwise setuptools[scm] doesn't seem to realize that the files should be included (not sure why it does realize the .py files should).

It is supposed to be a way to include them automatically if the sub-command build_py reports the new *.pyi files:

• https://setuptools.pypa.io/en/latest/userguide/extension.html#supporting-sdists-and-editable-installs-in-build-sub-commands

This is probably also needed to support editable installs.

[llucax]

This is much less pressing now because we are generating a MANIFEST.in with the cookiecutter templates.