The transition is done. Can you work on a patch and maybe add some
tests, too? It helps when you start Python with the -bb flag:

$ ./python -bb -c 'import imaplib; imaplib.Debug=5;
imaplib.IMAP4("mail.rtmq.infosathse.com")'
  52:01.86 imaplib version 2.58
  52:01.86 new IMAP4 connection, tag=PNFO
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/heimes/dev/python/py3k/Lib/imaplib.py", line 184, in __init__
    self.welcome = self._get_response()
  File "/home/heimes/dev/python/py3k/Lib/imaplib.py", line 907, in
_get_response
    resp = self._get_line()
  File "/home/heimes/dev/python/py3k/Lib/imaplib.py", line 1009, in
_get_line
    self._mesg('< %s' % line)
  File "/home/heimes/dev/python/py3k/Lib/warnings.py", line 62, in warn
    globals)
  File "/home/heimes/dev/python/py3k/Lib/warnings.py", line 102, in
warn_explicit
    raise message
BytesWarning: str() on a bytes instance