This is a new syntax added in python3.7, so black can't verify that reformatting will not change the ast unless black itself is run with 3.7. We'll need to change the error message black gives in this case. @ambv any ideas?
Fixes#125.
This support isn't *exactly* right per PEP 3131 as the regex engine is a bit
too limited for that and I didn't want to spend time on Other_ID_Start and
Other_ID_Continue unless they're actually needed.
Hopefully this doesn't slow it down too much.
