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")

{
    "url":"ws://127.0.0.1:8888",
    "realm":"Automation",
    "ID":"wamptest",
    "password":"1234567890",
    "key":"ABCDEFGHIJ"
}