Tiny Python 3k fix

[2013-07-09 13:42:12,034: ERROR/MainProcess] Process Worker-3
Traceback (most recent call last):
  File "python3.2/site-packages/billiard-3.3.0.0rc1-py3.2.egg/billiard/process.py", line 257, in _bootstrap
    logger_names.append(None)  # for root logger
AttributeError: 'dict_keys' object has no attribute 'append'

The return value of dict_keys() needs to be converted to a list explicitly.