Traceback when resetting Workbench perspectives in Python 3

To replicate, eg. start Mayavi and select the reset perspectives menu item.
```
Traceback (most recent call last):
File "/Users/cwebster/src/ets/pyface/pyface/ui/qt4/action/action_item.py", line 186, in _qt4_on_triggered
self.controller.perform(action, action_event)
File "/Users/cwebster/src/ets/pyface/pyface/workbench/action/action_controller.py", line 35, in perform
return action.perform(event)
File "/Users/cwebster/src/ets/pyface/pyface/workbench/action/reset_all_perspectives_action.py", line 36, in perform
window.reset_all_perspectives()
File "/Users/cwebster/src/ets/pyface/pyface/workbench/workbench_window.py", line 507, in reset_all_perspectives
for id in self._memento.perspective_mementos.keys():
RuntimeError: dictionary changed size during iteration
```