pygi-closure: Use new girepository closure api

Fixes #556 (closed) and probably also #535 (closed)

Replaced deprecated g_callable_info_prepare_closure and g_callable_info_free_closure with new APIs g_callable_info_create_closure and g_callable_info_destroy_closure. The pointer from to g_callable_info_create_closure is passed to g_callable_info_get_closure_native_address to obtain another pointer equivalent to the return value from the old prepare_closure API and then is saved to a new field in PyGICClosure. The value of the new field is used for freeing with g_callable_info_destroy_closure. The previous code called g_callable_info_free_closure which does nothing and leaks memory resulting in issue #556 (closed).