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-policy.rst

@@ -116,6 +116,10 @@ asyncio ships with the following built-in policies:
 
       On Windows, :class:`ProactorEventLoop` is now used by default.
 
+   .. versionchanged:: 3.12
+      :meth:`get_event_loop` now raises a :exc:`RuntimeError` if there is no
+      current event loop set.
+
 
 .. class:: WindowsSelectorEventLoopPolicy