Fix synchro test failures with PyMongo 3.12

 [2021/07/15 23:57:40.337] ERROR: Failure: ImportError (cannot import name '_TxnState' from 'synchro' (/data/mci/eb739280245a00d7b8911c9e443753d1/src/synchro/__init__.py))

 [2021/07/15 23:57:40.337] ----------------------------------------------------------------------

 [2021/07/15 23:57:40.337] Traceback (most recent call last):

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/failure.py", line 39, in runTest

 [2021/07/15 23:57:40.337]     raise self.exc_val.with_traceback(self.tb)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/loader.py", line 418, in loadTestsFromName

 [2021/07/15 23:57:40.337]     addr.filename, addr.module)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/importer.py", line 47, in importFromPath

 [2021/07/15 23:57:40.337]     return self.importFromDir(dir_path, fqname)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/importer.py", line 94, in importFromDir

 [2021/07/15 23:57:40.337]     mod = load_module(part_fqname, fh, filename, desc)

 [2021/07/15 23:57:40.337]   File "/opt/python/3.7/lib/python3.7/imp.py", line 234, in load_module

 [2021/07/15 23:57:40.337]     return load_source(name, filename, file)

 [2021/07/15 23:57:40.337]   File "/opt/python/3.7/lib/python3.7/imp.py", line 171, in load_source

 [2021/07/15 23:57:40.337]     module = _load(spec)

 [2021/07/15 23:57:40.337]   File "<frozen importlib._bootstrap>", line 696, in _load

 [2021/07/15 23:57:40.337]   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked

 [2021/07/15 23:57:40.337]   File "<frozen importlib._bootstrap_external>", line 728, in exec_module

 [2021/07/15 23:57:40.337]   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/test_change_stream.py", line 48, in <module>

 [2021/07/15 23:57:40.337]     from test.unified_format import generate_test_classes

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/unified_format.py", line 37, in <module>

 [2021/07/15 23:57:40.337]     from pymongo.client_session import ClientSession, TransactionOptions, _TxnState

 [2021/07/15 23:57:40.337] ImportError: cannot import name '_TxnState' from 'synchro' (/data/mci/eb739280245a00d7b8911c9e443753d1/src/synchro/__init__.py)

 [2021/07/15 23:57:40.337] ======================================================================

 [2021/07/15 23:57:40.337] ERROR: test_hashable (test.test_client.TestClient)

 [2021/07/15 23:57:40.337] ----------------------------------------------------------------------

 [2021/07/15 23:57:40.337] Traceback (most recent call last):

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/test_client.py", line 668, in test_hashable

 [2021/07/15 23:57:40.337]     self.assertIn(c, {client_context.client})

 [2021/07/15 23:57:40.337] TypeError: unhashable type: 'MongoClient'

 [2021/07/15 23:57:40.337] ======================================================================

 [2021/07/15 23:57:40.337] ERROR: test_hashable (test.test_collection.TestCollection)

 [2021/07/15 23:57:40.337] ----------------------------------------------------------------------

 [2021/07/15 23:57:40.337] Traceback (most recent call last):

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/test_collection.py", line 158, in test_hashable

 [2021/07/15 23:57:40.337]     self.assertIn(self.db.test.mike, {self.db["test.mike"]})

 [2021/07/15 23:57:40.337] TypeError: unhashable type: 'Collection'

...

 [2021/07/15 23:57:40.337] ERROR: test_hashable (test.test_database.TestDatabase)

 [2021/07/15 23:57:40.337] ----------------------------------------------------------------------

 [2021/07/15 23:57:40.337] Traceback (most recent call last):

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/test_database.py", line 131, in test_hashable

 [2021/07/15 23:57:40.337]     self.assertIn(self.client.test, {Database(self.client, "test")})

 [2021/07/15 23:57:40.337] TypeError: unhashable type: 'Database'

 [2021/07/15 23:57:40.337] ======================================================================

 [2021/07/15 23:57:40.337] ERROR: test suite for <class 'test.test_encryption.TestBsonSizeBatches'>

 [2021/07/15 23:57:40.337] ----------------------------------------------------------------------

 [2021/07/15 23:57:40.337] Traceback (most recent call last):

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/suite.py", line 210, in run

 [2021/07/15 23:57:40.337]     self.setUp()

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/suite.py", line 293, in setUp

 [2021/07/15 23:57:40.337]     self.setupContext(ancestor)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/suite.py", line 316, in setupContext

 [2021/07/15 23:57:40.337]     try_run(context, names)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/lib/python3.7/site-packages/nose/util.py", line 471, in try_run

 [2021/07/15 23:57:40.337]     return func()

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/test_encryption.py", line 1043, in setUpClass

 [2021/07/15 23:57:40.337]     auto_encryption_opts=opts, event_listeners=[cls.listener])

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/utils.py", line 536, in rs_or_single_client

 [2021/07/15 23:57:40.337]     return _mongo_client(h, p, **kwargs)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/test/utils.py", line 497, in _mongo_client

 [2021/07/15 23:57:40.337]     **client_options)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/synchro/__init__.py", line 353, in __init__

 [2021/07/15 23:57:40.337]     self.delegate = self.__delegate_class__(host, port, *args, **kwargs)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/motor/core.py", line 158, in __init__

 [2021/07/15 23:57:40.337]     delegate = self.__delegate_class__(*args, **kwargs)

 [2021/07/15 23:57:40.337]   File "/data/mci/eb739280245a00d7b8911c9e443753d1/src/.tox/synchro37/tmp/mongo-python-driver/pymongo/mongo_client.py", line 783, in __init__

 [2021/07/15 23:57:40.337]     from pymongo.encryption import _Encrypter

 [2021/07/15 23:57:40.338] ImportError: cannot import name '_Encrypter' from 'synchro' (/data/mci/eb739280245a00d7b8911c9e443753d1/src/synchro/__init__.py)