Weak references don't primarily exist to deal with cycles, and cycles generally often involve necessarily strong references even in systems that support weak references. (That is, e.g., cycles where A needs to be reachable if B is reachable and vice versa, such that a weak reference that allowed B to be disposed while A was reachable or vice versa would not be acceptable. A system that can detect cyclic garbage will still be able to collect both A and B -- connected by strong references -- when neither can be reached from the rest of the program.)
The standard use case for weak references in Objective-C is a child-to-parent reference. The parent holds a strong reference to the child and children hold weak references to their parents, avoiding potential cycles.
In fact, part of good memory management in a reference counted system is that you should always have a hierarchy to your data structures so it never makes sense to have an actual strong reference loop.
So then you play pass-the-strong-reference in a destructor - i.e. if A goes out of scope the A->B ref is weakened and the B->A ref is strengthened, then vice versa if required.
We did this for DBIx::Class in perl, thereby keeping full liveness but still getting timely destruction for connection objects. It was a trifle insane to get right, but it works extremely well.
Sort of, CPython has both reference counting and a normal Garbage Collection. You can disable the latter and still avoid memory leaks as long as you don't have cycles
https://docs.python.org/3/library/weakref.html