Some cosmetic changes

Changed decision tree for py2 / py3 to simpler one, and changed the regex to work simpler.

if you use it the same, i think it's simpler.
but you are right that in the future it could make some problems, if someone assumes it's one module, and find out it can be an other one.
this problem can be solved by assigning get_packages to the right method, instead of declaring it as a function, and import the modules with their real name.