False positives for traitsui.editors DeprecationWarning

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.