gh-93453: No longer create an event loop in get_event_loop() (#98440)

asyncio.get_event_loop() now always return either running event loop or
the result of get_event_loop_policy().get_event_loop() call. The latter
should now raise an RuntimeError if no current event loop was set
instead of creating and setting a new event loop.

It affects also a number of asyncio functions and constructors which
call get_event_loop() implicitly: ensure_future(), shield(), gather(),
etc.

DeprecationWarning is no longer emitted if there is no running event loop but
the current event loop was set.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

• Doc/library/asyncio-eventloop.rst[diff]
• Doc/library/asyncio-llapi-index.rst[diff]
• Doc/library/asyncio-policy.rst[diff]
• Doc/whatsnew/3.12.rst[diff]
• Lib/asyncio/events.py[diff]
• Lib/asyncio/futures.py[diff]
• Lib/asyncio/streams.py[diff]
• Lib/asyncio/tasks.py[diff]
• Lib/test/test_asyncio/test_base_events.py[diff]
• Lib/test/test_asyncio/test_events.py[diff]
• Lib/test/test_asyncio/test_futures.py[diff]
• Lib/test/test_asyncio/test_streams.py[diff]
• Lib/test/test_asyncio/test_tasks.py[diff]
• Lib/test/test_asyncio/test_unix_events.py[diff]
• Lib/test/test_coroutines.py[diff]
• Misc/NEWS.d/next/Library/2022-10-19-13-37-23.gh-issue-93453.wTB_sH.rst[Added - diff]
• Modules/_asynciomodule.c[diff]
• Modules/clinic/_asynciomodule.c.h[diff]