bpo-30098: Clarify that run_coroutine_threadsafe expects asyncio.Future (GH-1170)

Lib/asyncio/tasks.py

@@ -517,7 +517,8 @@ def ensure_future(coro_or_future, *, loop=None):
     elif compat.PY35 and inspect.isawaitable(coro_or_future):
         return ensure_future(_wrap_awaitable(coro_or_future), loop=loop)
     else:
-        raise TypeError('A Future, a coroutine or an awaitable is required')
+        raise TypeError('An asyncio.Future, a coroutine or an awaitable is '
+                        'required')
 
 
 @coroutine