Add black-primer unittests

[cooperlees]

• Get this tool covered with some decent unittests for all unittests wins
• Have a CLI and lib test class
• Import it from test_black.py so we always run tests
• Revert typing asyncio.Queue as Queue[str] so we can work in 3.6
• mypy: Until black > 3.6 disallow_any_generics=False for primer code

Test:

• Run tests: coverage run tests/test_primer.py or coverage run -m unittest

(b) cooper-mbp1:black cooper$ coverage report
Name                      Stmts   Miss  Cover
---------------------------------------------
src/black_primer/cli.py      49      8    84%
src/black_primer/lib.py     148     28    81%
tests/test_primer.py        114      1    99%
---------------------------------------------
TOTAL                       311     37    88%

Addresses #1390