adapt finalisation code to what CPython 3.6/7 does, fix error display if not-awaited warning turns into an error

move protocol error to the right spot to stop overwriting similar exceptions that are already created in __Pyx__Coroutine_GetAwaitableIter()

repair scoped comprehensions inside of generators and async functions by allowing their loop variables to be added to closures when necessary

integrate new Py3.6+ couroutine tests and repair misbehaviour in error cases and when async-iterating over awaitable tuple/StopIteration subtypes (however rare that is)

do not call PyObject_IsSubclass() with an exception set (instead rely on it being cleared when printing it before)

Disable asyncio coroutine tests in Py3.4 as coroutines are not iterables but Task() features this beautiful line of code:
        self._coro = iter(coro)  # Use the iterator just in case.

in generators/coroutines, save away the current exception in the 'return' case of finally clauses as 'return' actually raises an (Async)StopIteration exception

allow yielding exception instances from async generators (previously interpreted them as actual exceptions)