Detect quote style ignoring docstrings (#3306)
Currently the quote style of the first string in a file is used for autodetecting what to use when rewriting code for fixes. This is an okay heuristic, but often the first line in a file is a docstring, rather than a string constant, and it's not uncommon for pre-Black code to have different quoting styles for those.
For example, in the Google style guide:
https://google.github.io/styleguide/pyguide.html
> Be consistent with your choice of string quote character within a file. Pick ' or " and stick with it. ... Docstrings must use """ regardless.
This branch adjusts the logic to instead skip over any `"""` triple doublequote string tokens. The default, if there are no single quoted strings, is still to use double quote as the style.
