Break reference cycle between FilterLink and Graph

[lgeiger]

This breaks a reference cycle similar to what has been done for FilterContext in #1439.

import gc
from av.filter import Graph


def test_graph_leak():
    graph = Graph()
    src = graph.add("testsrc")
    sink = graph.add("buffersink")
    src.link_to(sink)
    src.outputs[0].link


test_graph_leak()

gc.set_debug(gc.DEBUG_SAVEALL)
collected = gc.collect()
print(f"Collected {collected} objects")
for obj in gc.garbage:
    print(f"  - {type(obj).__name__}")

main:

Collected 14 objects
  - Graph
  - dict
  - dict
  - dict
  - Filter
  - FilterContext
  - ReferenceType
  - list
  - Filter
  - FilterContext
  - list
  - FilterContextPad
  - tuple
  - FilterLink

This PR:

Collected 6 objects
  - Filter
  - FilterContext
  - ReferenceType
  - FilterContextPad
  - tuple
  - FilterLink

Found while looking into #2282