Add py.typed file.

[neighthan]

This seems to work based on my following quick test:

# make a file to check whether mypy finds type hints
cat > test.py << EOF
import black
EOF

With the current black version I get

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

After running pip install git+https://github.com/neighthan/black@patch-1 I see

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

I've also confirmed the file is installed where we'd expect:

$ ls ~/anaconda/lib/python3.7/site-packages/black
__init__.py  __pycache__/  py.typed

Fixes #1316

[cooperlees]

Wow - Fast turnaround! Thanks.