Python 3 deprecated the logger.warn method in favor of warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning, so we
prefer to use warning to avoid DeprecationWarning.

Change-Id: Ic4e07276def17ef39764601c8a7e0991f9a4efbc
Closes-Bug: 1508442