[3.10] gh-93453: Only emit deprecation warning in asyncio.get_event_loop when a new event loop is created (#100059)

It no longer emits a deprecation warning if the current event loop was set.

(cherry picked from commit 3fae04b10e2655a20a3aadb5e0d63e87206d0c67)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>