Return copies from _get_free/in_use_connections and fix async _mock (#3967)

Co-authored-by: petyaslavova <petya.slavova@redis.com>

Author: bysiber

redis/connection.py

@@ -3088,13 +3088,13 @@ def _get_pool_lock(self):
 
     def _get_free_connections(self):
         with self._lock:
-            return self._available_connections
+            return list(self._available_connections)
 
     def _get_in_use_connections(self):
         with self._lock:
-            return self._in_use_connections
+            return set(self._in_use_connections)
 
-    async def _mock(self, error: RedisError):
+    def _mock(self, error: RedisError):
         """
         Dummy functions, needs to be passed as error callback to retry object.
         :param error: