async_cluster: create asyncio.Lock inside async functions

This allows creating the client at the module level or before the event loop has been initialized.

redis/asyncio/cluster.py

@@ -315,11 +315,13 @@ def __init__(
             list(res.values())[0], **kwargs
         )
         self._initialize = True
-        self._lock = asyncio.Lock()
+        self._lock: Optional[asyncio.Lock] = None
 
     async def initialize(self) -> "RedisCluster":
         """Get all nodes from startup nodes & creates connections if not initialized."""
         if self._initialize:
+            if not self._lock:
+                self._lock = asyncio.Lock()
             async with self._lock:
                 if self._initialize:
                     try:
@@ -337,6 +339,8 @@ async def initialize(self) -> "RedisCluster":
     async def close(self) -> None:
         """Close all connections & client if initialized."""
         if not self._initialize:
+            if not self._lock:
+                self._lock = asyncio.Lock()
             async with self._lock:
                 if not self._initialize:
                     self._initialize = True