Fix incorrect type hints for txredis. (#12042)

Some properties were marked as RedisProtocol instead of ConnectionHandler,
which wraps RedisProtocol instance(s).

synapse/replication/tcp/external_cache.py

@@ -21,7 +21,7 @@ from synapse.logging.context import make_deferred_yieldable
 from synapse.util import json_decoder, json_encoder
 
 if TYPE_CHECKING:
-    from txredisapi import RedisProtocol
+    from txredisapi import ConnectionHandler
 
     from synapse.server import HomeServer
 
@@ -63,7 +63,7 @@ class ExternalCache:
     def __init__(self, hs: "HomeServer"):
         if hs.config.redis.redis_enabled:
             self._redis_connection: Optional[
-                "RedisProtocol"
+                "ConnectionHandler"
             ] = hs.get_outbound_redis_connection()
         else:
             self._redis_connection = None