Qt: Silence an exception at interpreter exit

This silences the following warning about an ignored exception:
```
Exception ignored while calling deallocator <function TimerQT.__del__ at 0x...>:
Traceback (most recent call last):
  File ".../lib/matplotlib/backends/backend_qt.py", line 198, in __del__
    self._timer_stop()
  File ".../lib/matplotlib/backends/backend_qt.py", line 210, in _timer_stop
    self._timer.stop()
RuntimeError: wrapped C/C++ object of type QTimer has been deleted
```

Author: QuLogic

lib/matplotlib/backends/backend_qt.py

@@ -195,7 +195,13 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     def __del__(self):
-        self._timer_stop()
+        try:
+            self._timer_stop()
+        except RuntimeError as e:
+            # Silence warning on shutdown.
+            ignore_msg = "wrapped C/C++ object of type QTimer has been deleted"
+            if str(e) != ignore_msg:
+                raise
 
     def _timer_set_single_shot(self):
         self._timer.setSingleShot(self._single)