False positives for traitsui.editors DeprecationWarning

[mdickinson]

There's currently a DeprecationWarning issued if one tries to import editors from traitsui.editors instead of from traitsui.editors.api. However, the criterion for issuing the warning is a little too broad: the check is if name in traitsui.editors.api.__dict__, and this picks up things like the __warningsregistry__ attribute. This results in the warning being issued unexpectedly (e.g., as a result of the warnings machinery accessing the traitsui.editors.__warningsregistry).

This may be a use-case for adding an __all__ to traits.editors.api. Or we could tighten up the check.