Termux's Python environment doesn't provide sem_open, but fails with a
nested `ImportError` on import attempts:
ImportError: cannot import name 'SemLock' from '_multiprocessing'
This updates the existing handling for AWS Lambda to catch both
`OSError` and `ImportError`.
- Move black.py to src/black/__init__.py
- Have setuptools_scm make src/_black_version.py and exclude from git
- Move blackd.py to src/blackd/__init__.py
- Move blib2to3/ to src/
- Update `setup.py`
- Update unittests to pass
- Mostly path fixing + resolving
- Update CI
- pre-commit config
- appveyor + travis
Tested on my mac with python 3.7.5 via:
```
python3 -m venv /tmp/tb3
/tmp/tb3/bin/pip install --upgrade setuptools pip coverage pre-commit
/tmp/tb2/bin/pip install ~/repos/black/
cd ~/repos/black/
/tmp/tb2/bin/coverage run tests/test_black.py
/tmp/tb3/bin/pre-commit run -a
/tmp/tb3/bin/black --help
/tmp/tb3/bin/black ~/repos/ptr/ptr.py
```
