[PyQt] PSA: don't use open() in generators in QThreads

[Andrew Suffield]

http://bugs.python.org/issue14432

Looks like python will crash if you do any of the operations affected
by restricted execution (create file objects, meddle with __dict__,
etc) from a generator that was created in an older iteration of the
event loop.

The main python thread should be safe, since its thread state doesn't
get shredded regularly.