Issue #20599: Force ASCII encoding for stdout in test_cleanup() of test_builtin

On Windows, the codec of sys.stdout is implemented in Python. At exit, the
codec may be unloaded before the destructor tries to write something to
sys.stdout.
whatsnew: LOAD_CLASSDEREF, -X showrefcount
whatsnew: PyUnicode_FromFormat width/precision, thread cleanup after fork.
whatsnew: 'other improvements' and 'optimizations' belong under CPython heading.
asyncio.events: Use __slots__ in Handle and TimerHandle
asyncio: Change as_completed() to use a Queue, to avoid O(N**2) behavior. Fixes issue #20566.