Moving to the new `python.el’ shipped with emacs >=24.3

The new python-mode supports ipython almost out of  the box (you need
to setup some variable. The documentation in the module says everything).

So  I simplified  a lot  my configuration  ditching support  for older
emacs  that  I seldom  use  (on  platform  with  no ipython  or  jedi,
presumably).

Also now I do not need external python-mode package :-D

Sage-mode 0.7 is incompatible so I deactivated it.