byteify-strings: add support for ignore comments Our simple token analysis is sometimes not clever enough, we need to be able to turn off our script for parts of the code. This change introduces three special comments: - `#no-py3-transform` to tell `byteify-strings` ignore the next line - `#py3-transform: off` to ignore everything until the end of the file - `#py3-transform: on` to stop ignoring The last two can be particularly useful within Python 2/3 compatibility files.