fix: Include py.typed in package data for mypy

[DenverCoder1]

The py.typed file hasn't been included in wheels published to PyPI. This means that with pip install table2ascii locally, mypy still doesn't recognize it as a typed library. Running mypy on a file with just import table2ascii in it results in this output from mypy:

error: Skipping analyzing "table2ascii": module is installed, but missing library stubs or py.typed marker  [import]
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)