pylab and gtk : the main windows close !

The reason is that when pylab has been imported, it is too

    > late to change back ends

Mostly. There is an experimental pylab command switch_backends, but
this is not expected to work switching between GUI backends. It could
be used, for example, in an interactive GUI session to make postscript
the default backend when generating a bunch of figures in batch, and
then to switch back to the GUI backend.