Hi,
the unit tests in mongo-python-driver 3.4 are failing on Debian Linux

Failing tests:
test_gridfs (unittest.loader._FailedTest) ... ERROR
test_gridfs_bucket (unittest.loader._FailedTest) ... ERROR
test_monitoring (unittest.loader._FailedTest) ... ERROR
test_read_preferences (unittest.loader._FailedTest) ... ERROR
test_replica_set_client (unittest.loader._FailedTest) ... ERROR

Traceback:
======================================================================
ERROR: test_read_preferences (unittest.loader.ModuleImportFailure)
----------------------------------------------------------------------
ImportError: Failed to import test module: test_read_preferences
Traceback (most recent call last):
File "/usr/lib/python2.7/unittest/loader.py", line 254, in _find_tests
module = self._get_module_from_name(name)
File "/usr/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
_import_(name)
File "/home/fede/pymongo/test/test_read_preferences.py", line 39, in <module>
from test.test_replica_set_client import TestReplicaSetClientBase
File "/home/fede/pymongo/test/test_replica_set_client.py", line 83, in <module>
class TestReplicaSetClient(TestReplicaSetClientBase):
File "/home/fede/pymongo/test/test_replica_set_client.py", line 163, in TestReplicaSetClient
@client_context.require_secondaries_count(1)
File "/home/fede/pymongo/test/_init_.py", line 378, in require_secondaries_count
sec_count = len(self.client.secondaries)
AttributeError: 'NoneType' object has no attribute 'secondaries'

Steps to reproduce:
git clone git://github.com/mongodb/mongo-python-driver.git pymongo && cd pymongo && tox -epy27

The same errors and traceback are generated using tox -epy35 and/or using sources from the 3.4.0 tarball.

Environment: Debian Sid
Python versions: 2.7.13 3.5.1