Python interpreter should be hard-coded in /usr/bin/hp-*

Currently (as of 11.04, and I suspect in 11.10), hplip will potentially break if you have a non-system Python executable on your PATH, e.g. using virtualenv or a custom-built Python. As per the Debian Python Policy (I can't find a similarly thorough document for Ubuntu),

> The preferred specification for the Python interpreter is /usr/bin/python or /usr/bin/pythonX.Y. This ensures that a Debian installation of python is used and all dependencies on additional python modules are met.

> Maintainers should not override the Debian Python interpreter using /usr/bin/env python or /usr/bin/env pythonX.Y. This is not advisable as it bypasses Debian's dependency checking and makes the package vulnerable to incomplete local installations of python.

I think this is reasonable, and also supported by the majority of the Python scripts in my /usr/bin directory.

dwf@barricade:~$ lsb_release -rd
Description: Ubuntu 11.04
Release: 11.04
dwf@barricade:~$ apt-cache policy hplip
hplip:
  Installed: 3.11.1-2ubuntu2
  Candidate: 3.11.1-2ubuntu2
  Version table:
 *** 3.11.1-2ubuntu2 0
        500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages
        100 /var/lib/dpkg/status
dwf@barricade:~$ grep '#!/usr/bin/env python' /usr/bin/hp-* |cut -d':' -f 1 |xargs dpkg -S
hplip: /usr/bin/hp-align
hplip: /usr/bin/hp-check
hplip: /usr/bin/hp-clean
hplip: /usr/bin/hp-colorcal
hplip: /usr/bin/hp-firmware
hplip: /usr/bin/hp-hpdio
hplip: /usr/bin/hp-info
hplip: /usr/bin/hp-levels
hplip: /usr/bin/hp-makeuri
hplip: /usr/bin/hp-pkservice
hplip: /usr/bin/hp-plugin
hplip: /usr/bin/hp-probe
hplip: /usr/bin/hp-query
hplip: /usr/bin/hp-scan
hplip: /usr/bin/hp-setup
hplip: /usr/bin/hp-testpage
hplip: /usr/bin/hp-timedate
hplip: /usr/bin/hp-unload