Don't define TrapObserver as Protocol

The changed implementation relies on inheritance of `__init__`, making
this no longer a clean "Protocol", but a base class.  While this is
apparently supported in Python 3.11, Python versions earlier than 3.11
would override the inherited `__init__` method, thereby breaking this
implementation.

See python/cpython#88970 for details

src/zino/trapd.py

@@ -3,7 +3,7 @@
 import logging
 from dataclasses import dataclass, field
 from ipaddress import ip_address
-from typing import Any, List, NamedTuple, Optional, Protocol, Set
+from typing import Any, List, NamedTuple, Optional, Set
 
 from pysnmp.carrier.asyncio.dgram import udp
 from pysnmp.entity import config
@@ -54,7 +54,7 @@ def __str__(self):
         return f"<Trap from {self.agent.device.name}: {variables}>"
 
 
-class TrapObserver(Protocol):
+class TrapObserver:
     """Defines a valid protocol for SNMP trap observers.
 
     A trap observer that directly subclasses this protocol can expect to be automatically registered by Zino as an