[3.13] gh-121621: Move asyncio_running_loop to private struct (GH-121939) (#121943)

gh-121621: Move asyncio_running_loop to private struct (GH-121939)

This avoids changing the ABI and keeps the field in the private struct.
(cherry picked from commit 81fd625b5c30cc6f417c93bad404923676ad8ca3)

Co-authored-by: Sam Gross <colesbury@gmail.com>