add py.typed

fixes #111 (closed)

this file is required otherwise type checkers such as pyright and mypy will just ignore the types and report an error:

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

see https://mypy.readthedocs.io/en/latest/error_code_list.html#check-that-import-target-can-be-found-import-untyped and https://peps.python.org/pep-0561/#packaging-type-information

(i have no idea why they thought it was a good idea to make types not work by default unless you add this file, which many library developers aren't aware of, especially since type checkers won't report this problem during development, only for downstream users.)