[3.13] GH-125789: fix `fut._callbacks` to always return a copy of callbacks (#125922) (#125976)

GH-125789: fix `fut._callbacks` to always return a copy of callbacks (#125922)

Fix `asyncio.Future._callbacks` to always return a copy of the internal list of callbacks to avoid mutation from user code affecting the internal state.

(cherry picked from commit cae853e3b44cd5cb033b904e163c490dd28bc30a)