add pep-561 marker file

[hoefling]

This PR adds PEP 561 support for watchgod. The rationale for this PR is that with the current 0.6 release, the type checks with mypy fail because watchgod doesn't report having type hints. To reproduce:

$ cat run.py  # the example from README
import asyncio
from watchgod.main import awatch

async def main() -> None:
   async for changes in awatch('/tmp'):
       print(changes)

asyncio.run(main())
$ pip install watchgod mypy
$ mypy run.py
run.py:2: error: Skipping analyzing 'watchgod.main': found module but no type hints or library stubs
run.py:2: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

The current workaround is to add the marker file manually:

$ touch $(python -c "import watchgod; print(watchgod.__path__[0])")/py.typed
$ mypy run.py
Success: no issues found in 1 source file

[codecov]

Codecov Report

Merging #58 into master will not change coverage.
The diff coverage is n/a.

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  Lines          280       280           
  Branches        29        29           
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  Hits           280       280           

[samuelcolvin]

thanks so much.

[samuelcolvin]

oh, sorry I forgot, you need to set zip_safe=False for this to work I believe, I'll sort that now.