fix: add slots

Author: phi-friday

src/typed_diskcache/utils/deque.py

@@ -68,6 +68,8 @@ def main() -> None:
                 # [3, 2, 1, 0, 0, -1, -2, -3]
     """
 
+    __slots__ = ("_cache", "_maxlen")
+
     def __init__(
         self,
         values: Iterable[_T] | None = None,