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/asyncio/tasks.py

@@ -582,7 +582,7 @@ def as_completed(fs, *, timeout=None):
     from .queues import Queue  # Import here to avoid circular import problem.
     done = Queue()
 
-    loop = events._get_event_loop()
+    loop = events.get_event_loop()
     todo = {ensure_future(f, loop=loop) for f in set(fs)}
     timeout_handle = None
 
@@ -668,7 +668,7 @@ def _ensure_future(coro_or_future, *, loop=None):
                             'is required')
 
     if loop is None:
-        loop = events._get_event_loop(stacklevel=4)
+        loop = events.get_event_loop()
     try:
         return loop.create_task(coro_or_future)
     except RuntimeError:
@@ -749,7 +749,7 @@ def gather(*coros_or_futures, return_exceptions=False):
     gather won't cancel any other awaitables.
     """
     if not coros_or_futures:
-        loop = events._get_event_loop()
+        loop = events.get_event_loop()
         outer = loop.create_future()
         outer.set_result([])
         return outer