Fix separator alignment

[aaronayres35]

This issue was observed when working on #1691, and again while working on addressing #1312.

Previously I remember having run the example code in #1312 without this issue... I recently updated to macOS Big Sur, but I am unsure if that is related. I do not know what changed to cause this, especially since it appears to be an off by one error (which somehow wasn't a problem before ?).
In any case, running the example code given in #1312 without this fix I see:

and with this change:

Checklist

• Add a news fragment if this PR is news-worthy for end users. (see docs/releases/README.rst)

[aaronayres35]

Looking closer at the screenshots it appears as though the separator below the "Below" label is unaffected (i.e. it was already in the right place before)
I suspect because it was already the last row in the Group

[aaronayres35]

Also, with the changes in this PR I see this for the ArrayEditor example:

[aaronayres35]

@mdickinson rahul mentioned you may still be on an older version of macOS. If that's the case and you get a chance could you run the example code:

from traits.api import *
from traitsui.api import *

class SeparatorTest(HasTraits):
    view = View(
        Label('Above'),
        Item('_'),
        Label('Below'),
        Item('_'),
        Group(Label('Caverns')),
        Item('_'),
        Group(Label('Dungeons')),
        Item('_'),
        Group(Label("Extra dungeons")),
        Item('_'),
        Label('Foundations')
    )

if __name__ == "__main__":
    separator = SeparatorTest()
    separator.configure_traits()

On both master / this branch to see what looks correct.

[mdickinson]

Here's what I see with PySide2 / Python 3.6 / EDM env on macOS 10.15.7.

First on master:

Then on this branch:

[mdickinson]

Sorry, that second image is wrong (I forgot that etstool doesn't make an editable install by default). Here's the correct image

In short: everything looks good on macOS 10.15.

[aaronayres35]

Thank you @mdickinson!

[rahulporuri]

I ran the script that generates screenshots locally on windows and everything looks good on this branch. Sorry for the glacial pace of PR review