PipelineTest:test_memory_usage fails with new urllib3 on some platforms

Disable memory usage test since it fails on some platforms with urllib3 v1.24.1

After an update of urllib3 from 1.15.1 to 1.24.1 this test started to fail with about an 50% increase in memory. After my initial analysis through `objgraph` I have seen that there was an increase in amount of tuple's being used by the test which was proportional to `num_maps * num_elements`.

I've also opened the issue urllib3/urllib3#1514