python: Keep runloop alive

previously we didn't keep a reference of the vtkPythonRunLoop inside the ParaT class after binding it to asyncio's loop. As a result, if the code was running from python (as opposed to pvpython) the loop was destroyed after the initialize step since no one else was holding a reference ot it.

This issue didn't appear in the ci because pvpython creates the runloop on the C++ side.