Add py.typed for static analysis with mypy

[ryu22e]

The budoux source code contains type hints, but the following error occurs when using mypy.

$ cat main.py
import budoux
parser = budoux.load_default_japanese_parser()
$ mypy main.py
main.py:1: error: Skipping analyzing "budoux": module is installed, but missing library stubs or py.typed marker
main.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)

I added py.typed file according to the URL in the error message above. After adding the file, the error no longer occurs as shown below.

$ mypy main.py
Success: no issues found in 1 source file

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[tushuhei]

Thanks for your PR!
For some reason, I'm still seeing the same error in the GitHub action.
https://github.com/google/budoux/runs/6648740861

I believe the first thing needs to be fixed around mypy is that the GitHub action does not stop even if mypy is showing an error. I made another issue to follow up on this point at #67.

[tushuhei]

Never mind, actually I found that it's a separate issue to yours.

[tushuhei]

Sorry about the confusion. I managed to reproduce the issue and confirmed that your change resolves that. Thanks for your contribution!