- 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'
```
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