lyxpak.py: Use `subprocess` for spawning scripts.

lyxpak.py: Use `subprocess` for spawning scripts.

`os.popen` is deprecated since Python-2.6. Also, this fixes the handling
of files with quotes in their names. The patch requires Python >= 2.7.

Furthermore, the patch executes the lyx2lyx script with the same
interpreter used for it, to maintain compatibility.

I also removed some lines related to Python-2.4, as it is no longer
supported anyway.