Deprecate the use of CHECK_INTERFACES for automatic interface checking.

[mdickinson]

[Builds on the commits in #1225; related to issue #1221, but doesn't completely resolve that issue]

Currently, if has_traits.CHECK_INTERFACES is nonzero, the @provides decorator performs interface checking at the point where the class is decorated.

This has a number of issues:

• the machinery is awkward to use: typically, the call to provides happens at import time, so someone who wants to make use of this needs to be careful about import order, setting has_traits.CHECK_INTERFACES before the imports they want checked happen
• there's significant processing happening at import time, which is not ideal
• the interface checking performed doesn't match the isinstance check that Instance(ISomeInterface) now does at trait assignment time (pre Traits 6.0.0, it did match)
• the machinery complicates the has_traits.py source, adding a circular dependency that has to be resolved via a local import

Longer term, if we do interface checking we likely want to do it statically.

This PR deprecates the interface-checking behaviour of provides, and issues a deprecation warning for any use of @provides with a nonzero CHECK_INTERFACES value. In a future Traits version (likely 7.0), we'd then remove the interface-checking call from provides altogether.

In a search of Traits-using code, I failed to find any code outside Traits that makes use of a non-zero CHECK_INTERFACES. I would expect the impact of this change to be negligible.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference) (Not relevant in this case)
• Update User manual (docs/source/traits_user_manual)
• Update type annotation hints in traits-stubs (Not relevant in this case)

[mdickinson]

Updated after a master merge. Though now that #1165 is in, I'm wondering whether some of those comparison_mode=ComparisonMode.identity annotations can be removed. @kitchoi ?

Sorry. Wrong PR. This comment was meant for #1229

[mdickinson]

Updated the description to add some rationale.

[kitchoi]

LGTM

(Operational bit: this needs #1225 to be merged first, and then master merged back in here.)