Don't run test_max_pool_size_with_leaked_request_massive with greenlets

Can restore once Gevent 1.0 lands, but 0.13.8 still deadlocks on this test on Mac; not PyMongo's bug.
Unused import
Merge remote-tracking branch 'upstream/master' into PYTHON-436

Conflicts:
	test/test_pooling_base.py
Merge branch 'PYTHON-436' into PYTHON-512
Merge branch 'PYTHON-512'