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")

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{

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    "url":"ws://127.0.0.1:8888",

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    "realm":"Automation",

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    "ID":"wamptest",

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    "password":"1234567890",

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    "key":"ABCDEFGHIJ"

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}