stdlib: add __slots__ (#14611)

stdlib/hmac.pyi

@@ -20,6 +20,7 @@ def new(key: bytes | bytearray, msg: ReadableBuffer | None, digestmod: _DigestMo
 def new(key: bytes | bytearray, *, digestmod: _DigestMod) -> HMAC: ...
 
 class HMAC:
+    __slots__ = ("_hmac", "_inner", "_outer", "block_size", "digest_size")
     digest_size: int
     block_size: int
     @property