Add py.typed to make isort PEP-561-compatible

[asfaltboy]

Problem: mypy can't find type hints when importing isort:

$ mypy .
my_module.py:1: error: Skipping analyzing 'isort': found module but no type hints or library stubs
my_module.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 608 source files)

Solution: add a py.typed file to make isort PEP-561-compatible
Alternatives: add isort to typeshed, or worser workarounds

Shamelessly snugged the above from psf/black#1316 , the fix can be as simple as psf/black#1395