bpo-32166: Drop Python 3.4 code from asyncio (#4612)

* Drop Python 3.4 code from asyncio

* Fix notes

* Add missing imports

* Restore comment

* Resort imports

* Drop Python 3.4-3.5 specific code

* Drop redunant check

* Fix tests

* Restore _COROUTINE_TYPES order

* Remove useless code
3 files changed