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

(cherry picked from commit 21b3e04)

Author: ambv

Lib/asyncio/base_events.py

@@ -459,7 +459,8 @@ def run_until_complete(self, future):
                 # local task.
                 future.exception()
             raise
-        future.remove_done_callback(_run_until_complete_cb)
+        finally:
+            future.remove_done_callback(_run_until_complete_cb)
         if not future.done():
             raise RuntimeError('Event loop stopped before Future completed.')