Use Logger.warning instead of Logger.warn

`Logger.warn` is deprecated
https://docs.python.org/3/library/logging.html#logging.Logger.warning

In the Python 2.7 documentation it's not even mention https://docs.python.org/2.7/library/logging.html#logging.Logger.warning

In the Python 2.6 documentation this method is present as `logging.warn` but not as `logging.Logger.warn`