In raw strings, a single backslash means a literal backslash. It is also used to escape quotes if it precedes them. This means it is impossible to change the quote type for strings that contain an unescaped version of the other quote type.
Fixes#100
* 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
