[SPARK-12960] [PYTHON] Some examples are missing support for python2

Without importing the print_function, the lines later on like ```print("Usage: direct_kafka_wordcount.py <broker_list> <topic>", file=sys.stderr)``` fail when using python2.*. Import fixes that problem and doesn't break anything on python3 either.

Author: Mark Grover <mark@apache.org>

Closes #10872 from markgrover/python2_compat.