Add py.typed file

[zanieb]

This is required for the types in this library to be checked when imported downstream.

Resolves the following error:

❯ mypy example.py
example.py:2: error: Skipping analyzing "cleo.helpers": module is installed, but missing library stubs or py.typed marker
example.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source files)

[zanieb]

Sorry! I see this exists in #254