Enum editor unable to display un-hashable types.

[midhun-pm]

The Enum editor is unable to display drop-down menus that contain unhashable types such as lists, sets, etc.
The cause of the issue is in in traitsui.helper when trying to create the inverse mapping.

Example: The following valid declaration will raise a TypeError

from traits.api import *


class TestClass(HasTraits):

    sets = Enum({1}, {2}, {3})


TestClass().configure_traits()