whisperback: extra setup.py commands fail

While working on #16936, I noticed that both the build_man and the build_gtkbuilderi18n commands fail:

$ python3 setup.py build_man
Traceback (most recent call last):
  File "setup.py", line 91, in <module>
    "clean": clean_i18n.clean_i18n,
  File "/usr/lib/python3.5/distutils/core.py", line 134, in setup
    ok = dist.parse_command_line()
  File "/usr/lib/python3.5/distutils/dist.py", line 472, in parse_command_line
    args = self._parse_command_opts(parser, args)
  File "/usr/lib/python3.5/distutils/dist.py", line 544, in _parse_command_opts
    raise DistutilsClassError(msg % cmd_class)
distutils.errors.DistutilsClassError: command class <class '__main__.build_man'> must provide 'user_options' attribute (a list of tuples)

I tested this with Python 3.5 and 3.7.

Original created by @segfault on 16937 (Redmine)