Fix and allow classes with missing metaclasses (#9136)

stubs/jmespath/jmespath/functions.pyi

@@ -16,6 +16,6 @@ def signature(*arguments: _Signature) -> Callable[[_F], _F]: ...
 class FunctionRegistry(type):
     def __init__(cls, name, bases, attrs) -> None: ...
 
-class Functions:
+class Functions(metaclass=FunctionRegistry):
     FUNCTION_TABLE: Any
     def call_function(self, function_name, resolved_args): ...