Black will cache already formatted files using their file size and
modification timestamp. The cache is per-user and will always be used
unless Black is used with --diff or with code provided via standard
input.
* 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
