Make NoDefaultSpecified and no_default_specified private

[kitchoi]

Closes #1379

This PR fixes the issue by making NoDefaultSpecified and no_default_specified private by naming convention. We believe they are not (actively) used outside of traits. If they are indeed used, this PR will at least make those breakage fail in a more obvious and trackable fashion, and we will get to know why and how it gets used, and then we can fix the breakage accordingly.

Note that if I change this line to if True::

traits/traits/trait_type.py

Line 297 in a68bc50

if default_value is not no_default_specified:

I would get a test failure for the test that involves TraitsUI:

======================================================================
ERROR: test_create_editor (traits.tests.test_enum.TestGui)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kchoi/Work/ETS/traits/traits/tests/test_enum.py", line 320, in test_create_editor
    with UITester().create_ui(obj):
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 117, in __enter__
    return next(self.gen)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/testing/tester/ui_tester.py", line 88, in create_ui
    ui = object.edit_traits(**ui_kwargs)
  File "/Users/kchoi/Work/ETS/traits/traits/has_traits.py", line 1736, in edit_traits
    return view.ui(
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/view.py", line 462, in ui
    ui.ui(parent, kind)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/ui.py", line 244, in ui
    self.rebuild(self, parent)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/toolkit.py", line 163, in ui_live
    ui_live.ui_live(ui, parent)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/ui_live.py", line 43, in ui_live
    _ui_dialog(ui, parent, BaseDialog.NONMODAL)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/ui_live.py", line 65, in _ui_dialog
    BaseDialog.display_ui(ui, parent, style)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/ui_base.py", line 278, in display_ui
    ui.owner.init(ui, parent, style)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/ui_live.py", line 225, in init
    self.add_contents(panel(ui), bbox)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/qt4/ui_panel.py", line 253, in panel
    content = ui._groups
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/ui.py", line 832, in _get__groups
    shadow_group = self.view.content.get_shadow(self)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/group.py", line 288, in get_shadow
    content.append(value.get_shadow(ui))
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/group.py", line 297, in get_shadow
    return ShadowGroup(shadow=self, content=content, groups=groups)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traitsui/group.py", line 546, in __init__
    self.shadow = shadow
traits.trait_errors.TraitError: Cannot modify the read only 'shadow' attribute of a 'ShadowGroup' object.

Instead of relying on this test which really belonged to TraitsUI, I added another one that would fail for the same reason.

Ideally there should be a test for TraitType.clone, but I struggle to understand how it is supposed to be used. From what I saw, it is predominantly used by TraitType.__call__ and _TraitMaker. The former is used in the metaclass and the latter is used by Trait... both are quite complex. Eventually I decided this can be an exercise for another day, the existing usage in TraitsUI deserves a test in Traits anyway and it would provide the protection we needed for the code we changed here.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual) : This NoDefaultSpecified is not mentioned in the user manual
• Update type annotation hints in traits-stubs

[mdickinson]

LGTM. Thank you!

[mdickinson]

If you have a moment, it may make sense to add a changelog entry right now so that we don't forget later. But it can also wait until later.

[kitchoi]

Yes it would make sense to update changelog together with the PR. I tentatively categorize this as a removal, because it is a removal from the public API.

Since we have not got the news fragments machinery here, I updated the changelog directly instead. I am slightly worried about merge conflicts, but it is typically trivial enough (albeit a bit annoying at times).

[mdickinson]

Yes, we'll deal with any merge conflicts.

Thank you!