_FEEDBACK_MODES advertised MODE_ECB/MODE_CFB/MODE_OFB as supported, but
_Cipher.__init__ then rejected every mode other than CBC/CTR with a
contradictory 'not supported by this cipher' error after they had
already passed the 'is supported' check. Prune _FEEDBACK_MODES to the
modes the cipher actually implements (CBC, CTR) so unsupported modes
get a single, accurate rejection, and drop the now-dead else branch.
Robert de Vries
Juliusz Sosinowicz