Add PEP-561 py.typed file to support type checking

[p7g]

Without this file, you get an error like this where you import the package when type checking with mypy:

error: Skipping analyzing "resemblyzer": module is installed, but missing library stubs or py.typed marker  [import-untyped]

This is because, according to PEP-561:

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing.

Seeing as this library already has many type annotations, it would be great to take advantage of them when using Resemblyzer from a project checked with mypy.

In this PR, I added that py.typed file and included it in the package_data in setup.py so it would be included in the distribution in the same way as pretrained.pt.