gh-126353: remove implicit creation of loop from asyncio.get_event_loop (#126354)

Remove implicit creation of loop from `asyncio.get_event_loop`. This is a step forward of deprecating the policy system of asyncio.

Lib/asyncio/events.py

@@ -668,7 +668,6 @@ class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
 
     class _Local(threading.local):
         _loop = None
-        _set_called = False
 
     def __init__(self):
         self._local = self._Local()
@@ -678,28 +677,6 @@ class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
 
         Returns an instance of EventLoop or raises an exception.
         """
-        if (self._local._loop is None and
-                not self._local._set_called and
-                threading.current_thread() is threading.main_thread()):
-            stacklevel = 2
-            try:
-                f = sys._getframe(1)
-            except AttributeError:
-                pass
-            else:
-                # Move up the call stack so that the warning is attached
-                # to the line outside asyncio itself.
-                while f:
-                    module = f.f_globals.get('__name__')
-                    if not (module == 'asyncio' or module.startswith('asyncio.')):
-                        break
-                    f = f.f_back
-                    stacklevel += 1
-            import warnings
-            warnings.warn('There is no current event loop',
-                          DeprecationWarning, stacklevel=stacklevel)
-            self.set_event_loop(self.new_event_loop())
-
         if self._local._loop is None:
             raise RuntimeError('There is no current event loop in thread %r.'
                                % threading.current_thread().name)
@@ -708,7 +685,6 @@ class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
 
     def set_event_loop(self, loop):
         """Set the event loop."""
-        self._local._set_called = True
         if loop is not None and not isinstance(loop, AbstractEventLoop):
             raise TypeError(f"loop must be an instance of AbstractEventLoop or None, not '{type(loop).__name__}'")
         self._local._loop = loop