Use of `filter` function incompatible with Python 3

The simple filter function was removed from being a builtin to being part of the standard library in python 3.

It is used as a builtin here: https://github.com/apache/avro/blob/master/lang/py/test/set_avro_test_path.py#L44

Therefore in order to make incremental progress in making lang/py python 3 compatible, we must remove the use of filter or import it from functools.