Block size difference in avro library on Python3 causes some AvroIO tests to fail.

Traceback (most recent call last):
 File "/home/robbe/workspace/beam/sdks/python/apache_beam/io/avroio_test.py", line 308, in test_split_points
 self.assertEquals(split_points_report[-10:], [(2, 1)] * 10)
AssertionError: Lists differ: [(10, 1), (10, 1), (10, 1), (10, 1), (10, 1[42 chars], 1)] != [(2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2[32 chars], 1)]
First differing element 0:
(10, 1)
(2, 1)
+ [(2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1), (2, 1)]
- [(10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1),
- (10, 1)]