Thank you, Christopher.  This ticket got away from me, and I had to
rediscover the solution you had proposed here when I worked on
[e94c8bc845].

I just tested the python script in this ticket, using the tip of
core-8-6-branch, and I can confirm that every time I press the button
the current time appears in Pasteboard Viewer.  So I think I can
close this one now.

Incidentally, in case you hadn't noticed this, I finally managed to
convince Ned Deily to embed Tcl.framework and Tk.framework inside of
Python.framework.  This means that you can move those frameworks aside
and replace them with symlinks to
and That allows testing tkinter
against the current tip of whatever version of Tk Python happens to
be using.  (Hopefully that will be 9.0.2 before too long!)