Change how venv path is modified in vim plugin (#804)
Change how venv path is modified in vim plugin (#804)

- 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'
```
psf/black