Add a py.typed file

[dmtucker]

Hey there 👋
I'm in the process of adding types to pytest-mypy, which integrates with xdist in part via the pytest_configure_node hook.
The type for the node parameter is available from xdist.workermanage, but mypy still emits the following error:

error: Skipping analyzing "xdist.workermanage": module is installed, but missing library stubs or py.typed marker  [import-untyped]
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

I see lots of typing work was done to xdist in 3.6.0 (#1065).
Could we add a py.typed file to allow downstreams to take advantage of it too?

[bluetech]

I already managed to forget the details, but at the time I wrote "The typings are not exposed as that would need some work on the public API". I think what I meant is that we should clearly delineate what is considered public/stable API (that should be properly typed and audited, and the typing should not break), and what is considered private.