python: Check "buffer" when writing to sys.stdout for python2 compat.

* lang/python/tests/support.py (print_data): Add check for buffer.

When running with something like make -C lang/python check verbose=2 the
test would fail under python2, because the file objects do not have a
buffer property.

• Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>