Optional trait type

[k2bd]

Closes #1298

This PR:

• Creates the Optional trait type as a shorthand for Union(None, ...)
• Changes exception messages in Union to refer to subclass names, e.g. Optional (let me know if this is controversial and I can change!)
• Adds xfail tests for Constant, Union, and Optional demonstrating behaviour discussed in Constant constraint not enforced in Union #1784, some of these may also be controversial and I can remove/alter them if so

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in stub files

[k2bd]

Sorry if I'm missing something obvious, I'm not sure I can link the error in tests to these changes. Additionally the failing test passes for me on my Ubuntu-on-WSL-on-Windows.

Update: from here I'm guessing it's an external (to this MR) issue

[mdickinson]

Sorry, @k2bd: there hasn't been much in the way of spare cycles for Traits maintenance recently. Thank you for the PR - I do hope to get to it soon.

Yes, there's at least one unrelated issue here, related to a new release of PySide 6. I'll investigate.

[k2bd]

@mdickinson no worries at all! I just got around to looking into the error yesterday after seeing it wasn't something immediately apparent at the time, and didn't want to keep anyone waiting too long if it was a subtle problem with what I wrote!

At the time, I couldn't find other workflow failures I had access to to see it was likely independent

[mdickinson]

I've opened #1787 for the current crop of CI failures. This looks like a TraitsUI + PySide 6.7.0 issue.