fixup! client: drop support for ZODB < 3.10

See commit d9ab77b8

neo/client/app.py

@@ -797,8 +797,6 @@ class Application(object):
                     data = tryToResolveConflict(oid, current_serial,
                         undone_tid, undo_data, data)
                 except ConflictError:
-                    data = None
-                if data is None:
                     raise UndoError('Some data were modified by a later ' \
                         'transaction', oid)
                 undo_serial = None

neo/tests/client/testClientApp.py

@@ -502,12 +502,10 @@ class ClientApplicationTests(NeoUnitTestBase):
         app = self.getApp()
         tid = self.makeTID()
         txn = self.makeTransactionObject()
-        def tryToResolveConflict(oid, conflict_serial, serial, data):
-            pass
         app.master_conn = Mock()
         conn = Mock()
         self.assertRaises(StorageTransactionError, app.undo, tid,
-            txn, tryToResolveConflict)
+            txn, failing_tryToResolveConflict)
         # no packet sent
         self.checkNoPacketSent(conn)
         self.checkNoPacketSent(app.master_conn)
@@ -612,7 +610,7 @@ class ClientApplicationTests(NeoUnitTestBase):
         def tryToResolveConflict(oid, conflict_serial, serial, data,
                 committedData=''):
             marker.append((oid, conflict_serial, serial, data, committedData))
-            return None
+            raise ConflictError
         # The undo
         txn = self.beginTransaction(app, tid=tid3)
         self.assertRaises(UndoError, app.undo, tid1, txn, tryToResolveConflict)