Fix exception in bolt handshake when log level <10

[robsdedude]

Fixes an UnboundLocalError that gets raised during the protocol handshake if the effective log level of the logger "neo4j.io" is < 10 (strictly less than logging.DEBUG).

...
  File "<path-to-python>/site-packages/neo4j/_sync/io/_pool.py", line 408, in _acquire
    return connection_creator()
  File "<path-to-python>/site-packages/neo4j/_sync/io/_pool.py", line 230, in connection_creator
    connection = self.opener(
  File "<path-to-python>/site-packages/neo4j/_sync/io/_pool.py", line 624, in opener
    return 
Bolt.open(
  File "<path-to-python>/site-packages/neo4j/_sync/io/_bolt.py", line 369, in open
    s, protocol_version, handshake, data = BoltSocket.connect(
  File "<path-to-python>/site-packages/neo4j/_sync/io/_bolt_socket.py", line 311, in connect
    agreed_version, handshake, response = s._handshake(
  File "<path-to-python>/site-packages/neo4j/_sync/io/_bolt_socket.py", line 271, in _handshake
    return agreed_version, handshake, response
UnboundLocalError: local variable 'handshake' referenced before assignment

[MaxAake]

LGTM!