[3.12] GH-125789: fix `fut._callbacks` to always return a copy of callbacks (GH-125922) (#125977)

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

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
(cherry picked from commit cae853e3b44cd5cb033b904e163c490dd28bc30a)