Update src/cegpy/trees/_staged.py

Updated typing

Co-authored-by: Aditi Shenvi <39489147+ashenvi10@users.noreply.github.com>

src/cegpy/trees/_staged.py

@@ -722,7 +722,7 @@ def _validate_hyperstage(initial_staging: List, hyperstage) -> bool:
         return True
 
     @staticmethod
-    def _validate_initial_staging_mutually_exclusive(initial_staging: List):
+    def _validate_initial_staging_mutually_exclusive(initial_staging: List) -> bool:
         if len(set(chain(*initial_staging))) != len(list(chain(*initial_staging))):
             return False
         return True