modernized README
modernized README

We now support only python 2.6 and newer. Don't list ctypes as a code
dependency, as it has been a standard module for a couple of years now.

git-svn-id: 9ab2475816adfb8420036182e3900ffc754382bd@368 516c0664-e049-0410-bc6e-8f9a6051a51d

MatthieuDartiailh/pyvisa,pyvisa/pyvisa,hgrecco/pyvisa,rubund/debian-pyvisa