- Check if black venv path is not already in `sys.path`
- Append (not insert) path so that black doesn't incorrectly import backports (e.g. `typing`)
Avoids this error if `typing` is present in venv:
```
Traceback (most recent call last):
File "<string>", line 56, in <module>
File "/home/josh/.virtualenvs/default/lib/python3.7/site-packages/black.py", line 19, in <module>
from typing import (
File "/home/josh/.virtualenvs/default/lib/python3.7/site-packages/typing.py", line 1356, in <module>
class Callable(extra=collections_abc.Callable, metaclass=CallableMeta):
File "/home/josh/.virtualenvs/default/lib/python3.7/site-packages/typing.py", line 1004, in __new__
self._abc_registry = extra._abc_registry
AttributeError: type object 'Callable' has no attribute '_abc_registry'
```
Since 18.6b0 was released, there has been a new option to skip string
normalization when Black is called, but it wasn't able to be specified
from within the vim plugin. This commit adds that functionality.
Tested with g:black_skip_string_normalization set to 0 (off) and 1 (on).
Neovim uses stdout for `msgpack` communication and the `subprocess` call for `virtualenv` was leaking that stream. Fix is to attach to a `subprocess.PIPE`.
