This removes all but one usage of the `regex` dependency. Tricky bits included:
- A bug in test_black.py where we were incorrectly using a character range. Fix also submitted separately in #2643.
- `tokenize.py` was the original use case for regex (#1047). The important bit is that we rely on `\w` to match anything valid in an identifier, and `re` fails to match a few characters as part of identifiers. My solution is to instead match all characters *except* those we know to mean something else in Python: whitespace and ASCII punctuation. This will make Black able to parse some invalid Python programs, like those that contain non-ASCII punctuation in the place of an identifier, but that seems fine to me.
- One import of `regex` remains, in `trans.py`. We use a recursive regex to parse f-strings, and only `regex` supports that. I haven't thought of a better fix there (except maybe writing a manual parser), so I'm leaving that for now.
My goal is to remove the `regex` dependency to reduce the risk of breakage due to dependencies and make life easier for users on platforms without wheels.
- 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
```
