Mayavi 4.7.0 under Python 3 misbehaves when setting preferences

The following appears in the log upon setting preferences:

```
Python 3.6.8 (default, Jan 14 2019, 11:02:34)
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> Exception occurred in traits notification handler.
Please check the log file for details.
```
```
Exception occurred in traits notification handler for object: , trait: preferences, old value: , new value:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/traits/trait_notifiers.py", line 394, in __call__
self.handler(*args)
File "/usr/lib/python3/dist-packages/apptools/preferences/preferences_helper.py", line 81, in _preferences_changed
self._preferences_changed_listener, self._get_path()
File "/usr/lib/python3/dist-packages/apptools/preferences/preferences_helper.py", line 112, in _get_path
raise SystemError('no preferences path, %s' % self)
SystemError: no preferences path,
ERROR|2019-07-01 14:55:54,473|Exception occurred in traits notification handler for object: , trait: preferences, old value: , new value:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/traits/trait_notifiers.py", line 394, in __call__
self.handler(*args)
File "/usr/lib/python3/dist-packages/apptools/preferences/preferences_helper.py", line 81, in _preferences_changed
self._preferences_changed_listener, self._get_path()
File "/usr/lib/python3/dist-packages/apptools/preferences/preferences_helper.py", line 112, in _get_path
raise SystemError('no preferences path, %s' % self)
SystemError: no preferences path,
```