format_func may be unexpected by some editors

[kitchoi]

Currently the RangeEditor_demo.py fails to launch, with the following error:

Traceback (most recent call last):
  File "examples/demo/Standard_Editors/RangeEditor_demo.py", line 97, in <module>
    demo.configure_traits()
  File "/Users/kchoi/.edm/envs/traitsui-test-3.6-pyqt/lib/python3.6/site-packages/traits/has_traits.py", line 2088, in configure_traits
    args,
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/toolkit.py", line 232, in view_application
    context, view, kind, handler, id, scrollable, args
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/view_application.py", line 84, in view_application
    context, view, kind, handler, id, scrollable, args
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/view_application.py", line 126, in __init__
    args=self.args,
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/view.py", line 462, in ui
    ui.ui(parent, kind)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/ui.py", line 246, in ui
    self.rebuild(self, parent)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/toolkit.py", line 162, in ui_live
    ui_live.ui_live(ui, parent)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_live.py", line 43, in ui_live
    _ui_dialog(ui, parent, BaseDialog.NONMODAL)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_live.py", line 65, in _ui_dialog
    BaseDialog.display_ui(ui, parent, style)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_base.py", line 284, in display_ui
    ui.owner.init(ui, parent, style)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_live.py", line 225, in init
    self.add_contents(panel(ui), bbox)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_panel.py", line 262, in panel
    _fill_panel(panel, content, ui)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_panel.py", line 295, in _fill_panel
    gp = _GroupPanel(item, ui, suppress_label=True)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_panel.py", line 601, in __init__
    layout = self._add_items(content, inner)
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/qt4/ui_panel.py", line 854, in _add_items
    ui, object, name, item.tooltip, None
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/editors/range_editor.py", line 305, in custom_editor
    ui, object, name, description, parent
  File "/Users/kchoi/Work/ETS/traitsui/traitsui/editor_factory.py", line 155, in custom_editor
    invalid_trait_name=self.invalid,
TypeError: CustomEnumEditor() got an unexpected keyword argument 'format_func'

Looks related to #859

The example seems to work fine with v7.0.0.

[ievacerny]

Ah, that's because CustomEnumEditor in both implementations of range_editor.py is a function and not a class inheriting from Editor. SimpleEnumEditor in range_editor.py should have the same problem.