All __nonzero__ are methods that return bool (#9139)

stubs/peewee/peewee.pyi

@@ -1534,7 +1534,7 @@ class ModelBase(type):
     def __contains__(self, key): ...
     def __len__(self) -> int: ...
     def __bool__(self) -> bool: ...
-    __nonzero__: Incomplete
+    def __nonzero__(self) -> bool: ...
     def __sql__(self, ctx): ...
 
 class _BoundModelsContext(_callable_context_manager):