installation: lxml instead of pyRXP

* Removes pyRXP from the packages installed but will still pick as first
  choice as lxml doesn't have quite the same behaviour w.r.t. errors.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
global: 4suite removal

* Removes 4suite.

* Assumes that lxml is always present (because it must).

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>