There isn't a problem in most cases, since functions that take a callback usually call it before returning. If the library does keep a long-lived reference to a callback (e.g. a console control handler in Windows), there are ways to support this, depending on the context (e.g. global variable, class attribute, instance attribute, a dict). 

However, for the case where a callback should remain referenced for the lifetime of the process, it would be convenient to have a `callback_incref` option. Internally this would use a new function flag `FUNCFLAG_CALLBACK_INCREF`.