asyncio: sync with Tulip

* Fix pyflakes warnings: remove unused imports and variables
* asyncio.test_support now uses test.support and test.script_helper if available
Issue #22926: In debug mode, call_soon(), call_at() and call_later() methods of
asyncio.BaseEventLoop now use the identifier of the current thread to ensure
that they are called from the thread running the event loop.

Before, the get_event_loop() method was used to check the thread, and no
exception was raised when the thread had no event loop. Now the methods always
raise an exception in debug mode when called from the wrong thread. It should
help to notice misusage of the API.