Stop using deprecated log.warn()
Stop using deprecated log.warn()

I missed this in my previous cleanup back in 9221fcf83528b5c3657e43636cb84c1d18025acd.

dpkp/kafka-python,ohmu/kafka-python,Aloomaio/kafka-python,DataDog/kafka-python,wikimedia/operations-debs-python-kafka,scrapinghub/kafka-python,wikimedia/operations-debs-python-kafka,scrapinghub/kafka-python,Aloomaio/kafka-python,dpkp/kafka-python,Yelp/kafka-python,Yelp/kafka-python,mumrah/kafka-python,ohmu/kafka-python,mumrah/kafka-python