Fix a bad Instance trait declaration for a private lock

[mdickinson]

This PR fixes a bad Instance declaration that effectively declared that a _lock trait could only be assigned a built-in function or method (and in particular, could not be assigned a lock).

I really wanted to rename the events trait to _events too as part of this PR, but I can't be absolutely sure that it's not being used elsewhere. (And it is used in the tests, which isn't a great sign.)

No regression test, because I couldn't find a way to exercise the issue using only the public API, and I don't really want to add tests that test internal implementation details. However, if/when we deprecate Instance(some_instance), this code will now fail to give a DeprecationWarning, where the code in master would have given such.

Fixes #1408.

Checklist

• Tests. No test provided. Sorry.
• [ ] Update API reference (docs/source/traits_api_reference). N/A
• [ ] Update User manual (docs/source/traits_user_manual). N/A
• [ ] Update type annotation hints in traits-stubs. N/A

[mdickinson]

Whoops. Bad branch.