Add `py.typed` file

[mxr]

See below:

$ tail example.py
import ynab_sdk
$ mypy example.py
example.py:1: error: Skipping analyzing "ynab_sdk": module is installed, but missing library stubs or py.typed marker  [import-untyped]
example.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)
$ pip freeze | rg ynab
ynab-sdk==0.5.0

While adding stubs in typeshed is one solution, these have to be manually kept up-to-date. Instead, this project can add a py.typed file so that other Python code (which imports this project) can benefit from end-to-end typechecking