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>

Lib/test/test_coroutines.py

@@ -2418,7 +2418,8 @@ class UnawaitedWarningDuringShutdownTest(unittest.TestCase):
     def test_unawaited_warning_during_shutdown(self):
         code = ("import asyncio\n"
                 "async def f(): pass\n"
-                "asyncio.gather(f())\n")
+                "async def t(): asyncio.gather(f())\n"
+                "asyncio.run(t())\n")
         assert_python_ok("-c", code)
 
         code = ("import sys\n"