Wx `TreeEditor` extended selection mode no longer works

Traceback:
```
Traceback (most recent call last):
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/view_application.py", line 141, in OnInit
args=self.args,
File "/Users/cwebster/src/clean/traitsui/traitsui/view.py", line 450, in ui
ui.ui(parent, kind)
File "/Users/cwebster/src/clean/traitsui/traitsui/ui.py", line 234, in ui
self.rebuild(self, parent)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/toolkit.py", line 118, in ui_live
ui_live.ui_live(ui, parent)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_live.py", line 47, in ui_live
_ui_dialog(ui, parent, BaseDialog.NONMODAL)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_live.py", line 81, in _ui_dialog
BaseDialog.display_ui(ui, parent, style)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_base.py", line 61, in display_ui
ui.owner.init(ui, parent, style)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_live.py", line 195, in init
sw = panel(ui, window)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_panel.py", line 266, in panel
panel, content[0], ui
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_panel.py", line 423, in fill_panel_for_group
panel, group, ui, suppress_label, is_dock_window, create_panel
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_panel.py", line 577, in __init__
self.add_items(content, panel, self.sizer)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/ui_panel.py", line 932, in add_items
editor.prepare(item_panel)
File "/Users/cwebster/src/clean/traitsui/traitsui/editor.py", line 250, in prepare
self.init(parent)
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/tree_editor.py", line 103, in init
style = self._get_style()
File "/Users/cwebster/src/clean/traitsui/traitsui/wx/tree_editor.py", line 316, in _get_style
style |= wx.TR_MULTIPLE | wx.TR_EXTENDED
AttributeError: module 'wx' has no attribute 'TR_EXTENDED'
```