WhisperBack does not start on Jessie with French locales

Seems like the string encoding handling wasn’t fully ported to the (much saner) Python 3 way of doing things:

amnesia@amnesia:~$ whisperback 
Traceback (most recent call last):
  File "/usr/bin/whisperback", line 33, in <module>
    ui = whisperBack.WhisperBackUI()
  File "/usr/lib/python3/dist-packages/whisperBack/gui.py", line 145, in __init__
    self.backend = whisperBack.whisperback.WhisperBack()
  File "/usr/lib/python3/dist-packages/whisperBack/whisperback.py", line 108, in __init__
    self.__load_conf(os.path.join("/", "etc", "whisperback", "config.py"))
  File "/usr/lib/python3/dist-packages/whisperBack/whisperback.py", line 145, in __load_conf
    exec(code, self.__dict__)
  File "<string>", line 76, in <module>
TypeError: unsupported operand type(s) for %: 'bytes' and 'str'

Related issues

• Related to #7755 (closed)

Original created by @intrigeri on 10577 (Redmine)