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`.