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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                  <xsd:element name="Xenomai">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                      <xsd:attribute name="XenoConfig" type="xsd:string" use="optional" default="xeno-config"/>

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                    </xsd:complexType>

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                  </xsd:element>