Need to define ResolvedSerial ourselves

Because ZODB 5 no-longer does

src/ZEO/tests/ZEO4/StorageServer.py

@@ -43,13 +43,14 @@ from ZEO.Exceptions import AuthError
 from .monitor import StorageStats, StatsServer
 from .zrpc.connection import ManagedServerConnection, Delay, MTDelay, Result
 from .zrpc.server import Dispatcher
-from ZODB.ConflictResolution import ResolvedSerial
 from ZODB.loglevels import BLATHER
 from ZODB.POSException import StorageError, StorageTransactionError
 from ZODB.POSException import TransactionError, ReadOnlyError, ConflictError
 from ZODB.serialize import referencesf
 from ZODB.utils import oid_repr, p64, u64, z64
 
+ResolvedSerial = b'rs'
+
 logger = logging.getLogger('ZEO.StorageServer')
 
 def log(message, level=logging.INFO, label='', exc_info=False):