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.
(binary:application/octet-stream)