Fix typing protocol for py 3.7

Author: schroeder-

asyncua/pubsub/protocols.py

@@ -4,9 +4,15 @@
 from asyncua.ua.uatypes import Byte, ExtensionObject, String, UInt16, UInt32, UInt64
 from asyncua import ua
 from .uadp import UadpNetworkMessage
-from typing import List, Optional, Protocol, Union
+from typing import List, Optional, Union
 from .dataset import DataSetMeta, DataSetValue, PublishedDataSet
-
+try:
+    from typing import Protocol
+except ImportError:
+    # Protocol is only supported in Python >= 3.8
+    # if mypy support is needed we should add typing extension as requirement
+    class Protocol:
+        pass
 
 class PubSubSender(Protocol):
     """

asyncua/pubsub/uadp.py

@@ -35,8 +35,16 @@
 from asyncua.ua.status_codes import StatusCodes
 from asyncua.ua import VariantType
 from enum import IntEnum, IntFlag
-from typing import Optional, Protocol, Tuple, Union, List
+from typing import Optional, Tuple, Union, List
 from dataclasses import dataclass, field
+try:
+    from typing import Protocol
+except ImportError:
+    # Protocol is only supported in Python >= 3.8
+    # if mypy support is needed we should add typing_extension as requirement
+    class Protocol:
+        pass
+
 
 logger = logging.getLogger(__name__)