Add topology_description to MotorClient

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FAIL [0.002s]: test_client_attrs (tornado_tests.test_motor_core.MotorCoreTest)

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Traceback (most recent call last):

  File "/data/mci/0e3bfcb99e16d17a605ae4887dec7df8/src/.tox/py3-pymongo-v3.12/lib/python3.6/site-packages/tornado/testing.py", line 125, in __call__

    result = self.orig_method(*args, **kwargs)

  File "/data/mci/0e3bfcb99e16d17a605ae4887dec7df8/src/test/tornado_tests/test_motor_core.py", line 90, in test_client_attrs

    attrs(self.cx) - motor_client_only)

AssertionError: Items in the first set but not the second:

'topology_description'

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