Replace deprecated logging.warn with logging.warning

[hugovk]

Replace logging.warn (deprecated in Python 2.7, 2011) with logging.warning (added in Python 2.3, 2003).

• https://docs.python.org/3/library/logging.html#logging.Logger.warning
• https://github.com/python/cpython/issues/57444

[hugovk]

Re: https://github.com/klen/mixer/issues/191, closing this year-old PR, happy to re-open if there's interest. 👍