[3.6] call remove_done_callback in finally section (GH-1688) (#1755)

(cherry picked from commit 21b3e04c13)

Lib/asyncio/base_events.py

@@ -459,6 +459,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                 # local task.
                 future.exception()
             raise
+        finally:
             future.remove_done_callback(_run_until_complete_cb)
         if not future.done():
             raise RuntimeError('Event loop stopped before Future completed.')