fix hangs on mouse selection in case if wxPython uses wxWidgets with GTK3+ support

This is default configuration on Debian Buster+ and Ubuntu 18.04+.
Probably it'll fix completely problem with 'clipboard already open'
described in the previous commit 5ce6d08f ("make clipboard open minimal time as wxPython documentation recommends")

.. Beremiz documentation master file

Beremiz's documentation
=======================

Contents:

.. toctree::
   :maxdepth: 2

   overview
   manual/index
   standards