modules: make ReturnValue.new_objects covariant

Otherwise we have to do a lot of casting to ensure that we get the
appropriate invariant type

mesonbuild/modules/__init__.py

@@ -185,7 +185,7 @@ def is_module_library(fname):
 
 
 class ModuleReturnValue:
-    def __init__(self, return_value: T.Optional['TYPE_var'], new_objects: T.List['TYPE_var']) -> None:
+    def __init__(self, return_value: T.Optional['TYPE_var'], new_objects: T.Sequence['TYPE_var']) -> None:
         self.return_value = return_value
         assert isinstance(new_objects, list)
         self.new_objects = new_objects