re-add alias to keep API

Author: alexrudd2

pymodbus/transaction.py

@@ -476,3 +476,6 @@ def reset(self):
         self.transactions = type(  # pylint: disable=attribute-defined-outside-init
             self.transactions
         )()
+
+class DictTransactionManager(ModbusTransactionManager):
+    """Old alias for ModbusTransactionManager."""