Implicit sessions are not always discarded after network errors

FAIL: test_sessions_dirty_session_errors_Dirty_implicit_session_is_discarded_(non-cursor_returning_read) (test.test_session.TestSpec)

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Traceback (most recent call last):

  File "/Users/shane/git/mongo-python-driver/test/__init__.py", line 444, in wrap

    return f(*args, **kwargs)

  File "/Users/shane/git/mongo-python-driver/test/__init__.py", line 444, in wrap

    return f(*args, **kwargs)

  File "/Users/shane/git/mongo-python-driver/test/test_session.py", line 1395, in run_scenario

    self.run_scenario(scenario_def, test)

  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 641, in run_scenario

    self.run_test_ops(sessions, collection, test)

  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 547, in run_test_ops

    self.run_operations(sessions, collection, test['operations'])

  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 458, in run_operations

    self._run_op(sessions, collection, op, in_with_transaction)

  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 448, in _run_op

    result = self.run_operation(sessions, collection, op.copy())

  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 393, in run_operation

    result = cmd(**dict(arguments))

  File "/Users/shane/git/mongo-python-driver/test/test_session.py", line 1373, in assert_different_lsid_on_last_two_commands

    self.assertNotEqual(event1.command['lsid'], event2.command['lsid'])

AssertionError: {'id': Binary(b'PY\x1f247@\x7f\x890!\x90\xd0\xfe\xb1\\', 4)} == {'id': Binary(b'PY\x1f247@\x7f\x890!\x90\xd0\xfe\xb1\\', 4)}