setup: don't pass None to cmdclass.
setup: don't pass None to cmdclass.

Fixes:

    $ python setup.py --help-commands
    ...
    Extra commands:
    Traceback (most recent call last):
      File "setup.py", line 219, in <module>
        distutils.core.setup(**setup_options)
      File "/usr/lib/python2.7/distutils/core.py", line 137, in setup
        ok = dist.parse_command_line()
      File "/usr/lib/python2.7/distutils/dist.py", line 464, in parse_command_line
        if self.handle_display_options(option_order):
      File "/usr/lib/python2.7/distutils/dist.py", line 669, in handle_display_options
        self.print_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 745, in print_commands
        max_length)
      File "/usr/lib/python2.7/distutils/dist.py", line 706, in print_command_list
        klass = self.get_command_class(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 830, in get_command_class
        raise DistutilsModuleError("invalid command '%s'" % command)
    distutils.errors.DistutilsModuleError: invalid command 'bdist_wheel'

jwilk/pydiatra,jwilk/pydiatra