* 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
Now Black properly splits standalone comments within bracketed expressions.
They are treated as another type of split instead of being bolted on with
whitespace prefixes.
A related fix: now multiple comments might appear after a given leaf.
Fixes#22