* Normalize string quotes
Convert single-quoted strings to double-quoted. Convert triple single-quoted strings to triple double-quoted. Do not touch any strings where conversion would increase the number of backslashes.
Fixes#51.
* reformat Black itself
Trailing commas after * or ** in a function signature are only safe for Python 3.6
code. So now Black checks whether the file was already Python 3.6 to begin
with. If so, trailing commas are used in such cases. Otherwise, they're not.
When * and ** don't appear in a function signature, the trailing comma is
always safe.
Fixes#8