Add a test to check for good change handler types

[kitchoi]

This PR adds a small test to check a few different types of change handlers can be used with observe.

Not sure why would any of the builtin get used with observe in real projects, but they do work now (as long as they accept one argument). From the experience of manual testing, being able to use print (or just any callable accepting one positional argument) with observe was quite handy. If it should stop being supported, at least the test will alert us.

Note: repr could not be used with on_trait_change.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs

[mdickinson]

LGTM!