chore: Fix typing for InMemoryCache (#566)

Author: allenporter

roborock/devices/cache.py

@@ -47,7 +47,8 @@ async def set(self, value: CacheData) -> None:
 class InMemoryCache(Cache):
     """In-memory cache implementation."""
 
-    def __init__(self):
+    def __init__(self) -> None:
+        """Initialize the in-memory cache."""
         self._data = CacheData()
 
     async def get(self) -> CacheData: