Add and distribute py.typed file

[Rogdham]

The py.typed file must be distributed (according to PEP 561) to indicate to type checkers that the package suport type hints.

This PR adds the file, but I don't know how you package your library, so make sure that the py.typed file is in the resulting wheel and source build as well.

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As an illustration, given the following input file:

from umsgpack import packb
from typing import reveal_type

data = packb({"hello": "world!"})
reveal_type(data)

…here is the output of mypy (version 1.14.1) currently:

foo.py:1: error: Skipping analyzing "umsgpack": module is installed, but missing library stubs or py.typed marker  [import-untyped]
foo.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
foo.py:5: note: Revealed type is "Any"
Found 1 error in 1 file (checked 1 source file)

…whereas here is the output if the py.typed file would have been present:

foo.py:5: note: Revealed type is "builtins.bytes"
Success: no issues found in 1 source file