bpo-36613: call remove_done_callback if exception (GH-12800)

Call remove_done_callback() in finally block.

https://bugs.python.org/issue36613
(cherry picked from commit c1964e9)

Co-authored-by: gescheit <[email protected]>

Author: miss-islington

Lib/asyncio/tasks.py

@@ -457,10 +457,11 @@ def _on_completion(f):
     finally:
         if timeout_handle is not None:
             timeout_handle.cancel()
+        for f in fs:
+            f.remove_done_callback(_on_completion)
 
     done, pending = set(), set()
     for f in fs:
-        f.remove_done_callback(_on_completion)
         if f.done():
             done.add(f)
         else:

Misc/NEWS.d/next/Library/2019-04-12-13-52-15.bpo-36613.hqT1qn.rst

@@ -0,0 +1 @@
+Fix :mod:`asyncio` wait() not removing callback if exception