[3.12] gh-106503: asyncio._SelectorSocketTransport: fix cyclic reference on close(). (GH-106504) (#106514)

gh-106503: asyncio._SelectorSocketTransport: fix cyclic reference on close(). (GH-106504)
(cherry picked from commit 3e5ce7968f5ab715f649e296e1f6b499621b8091)

Co-authored-by: Andrew Geng <pteromys@gmail.com>