legacy: xmlmarclint import fix

* Moves import of sys module into main() function for things
  to magically work.

* Removes legacy "version" parameter.

* General PEP8/PEP257 fixes.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
docs: missing mkdir command addition

* Adds command `$ mkdir -p $HOME/src` to installation tutorial.

Reported-by: Kevin Bowrin <kjbowrin@gmail.com>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>