change return type to grouping

Author: Kei

pandas/core/groupby/grouper.py

@@ -669,7 +669,7 @@ def groups(self) -> dict[Hashable, Index]:
         return self._index.groupby(cats)
 
     @cache_readonly
-    def observed_grouping(self) -> Grouper:
+    def observed_grouping(self) -> Grouping:
         if self._observed:
             return self