PYTHON-2082 Unpin session after RetryableWriteErrors from commitTransaction

[ShaneHarvey]

PYTHON-2154 PYTHON-2189 Remove 4.5 transaction test workarounds

Explanation: The StaleConfig change in PYTHON-2189 accidentally causes the test_transactions suite to ignore any failed test because there was a missing raise statement. Now that we're no longer masking test failures, this test began to fail:

ERROR: test_transactions_mongos_recovery_token_commitTransaction_retry_succeeds_on_new_mongos (test.test_transactions.TestTransactions)
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Traceback (most recent call last):
  File "/Users/shane/git/mongo-python-driver/test/__init__.py", line 442, in wrap
    return f(*args, **kwargs)
  File "/Users/shane/git/mongo-python-driver/test/__init__.py", line 442, in wrap
    return f(*args, **kwargs)
  File "/Users/shane/git/mongo-python-driver/test/__init__.py", line 442, in wrap
    return f(*args, **kwargs)
  File "/Users/shane/git/mongo-python-driver/test/test_transactions.py", line 418, in run_scenario
    self.run_scenario(scenario_def, test)
  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 584, in run_scenario
    self.run_test_ops(sessions, collection, test)
  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 497, in run_test_ops
    self.run_operations(sessions, collection, test['operations'])
  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 403, in run_operations
    result = self.run_operation(sessions, collection, op.copy())
  File "/Users/shane/git/mongo-python-driver/test/utils_spec_runner.py", line 346, in run_operation
    result = cmd(**dict(arguments))
  File "/Users/shane/git/mongo-python-driver/pymongo/client_session.py", line 615, in commit_transaction
    _reraise_with_unknown_commit(exc)
  File "/Users/shane/git/mongo-python-driver/pymongo/client_session.py", line 597, in commit_transaction
    self._finish_transaction_with_retry("commitTransaction")
  File "/Users/shane/git/mongo-python-driver/pymongo/client_session.py", line 655, in _finish_transaction_with_retry
    return self._client._retry_internal(True, func, self, None)
  File "/Users/shane/git/mongo-python-driver/pymongo/mongo_client.py", line 1406, in _retry_internal
    raise last_error
WriteConcernError: Replication is being shut down

This test failure is caused by PYTHON-2082. In particular, we neglected to unpin the session on retryable commitTransaction errors. To fix the bug we unpin the session on all errors with the RetryableWriteError label.

I've already tested this change here: https://evergreen.mongodb.com/version/5efb9809c9ec447c47b57365

[prashantmital]

LGTM