From the duplicate (https://bugs.python.org/issue35881) :

After some hours of debugging, I have identified the (possible) cause of the majority of the refleaks. 

The type_comments created in ast.c with new_type_comment() never reaches 0 reference counts. This **seems** to be because the cleanup for the stmt_ty elements where the type_comments are included never call DECREF on these elements when destroyed unless I am missing something fundamental.