PEP-561 compliance: Mark jtd as typed with py.typed file

[ChristopherChudzicki]

When typechecking code that uses jtd with Mypy, Mypy complains that the package is untyped:

error: Skipping analyzing "jtd": 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 13 source files)

Mypy version 1.0.0, Python 3.11.2

Looking at the source code in this repository, it clearly has type annotations, but Mypy isn't picking them up.

Based on https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages, I think the project should have a py.typed file adjacent to setup.py. (And setup.py should mention it as part of package_data).

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Thanks for jtd; I've been enjoying it!