python: pakfire: Keep a reference to the Python context

python: pakfire: Keep a reference to the Python context

Since we are registering all sorts of callbacks to here, we cannot
deallocate the Ctx object, or we won't be able to use them any more.

This is being fixed by holding a reference to the Python object instead
of the context itself. That will at least help us with lots of these
problems.

There is still, however, the problem that both the Python Ctx and
Pakfire object can be dereferenced when the backend objects will remain
existing.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>