Fix version detection for all Python versions

Fix version detection for all Python versions

Inspecting sys.version_info using named component attributes was added in
the patch "Some fixes to start support for python3."

Named component attributes were only added to sys.version_info in Python
2.7 so their use breaks Cura for Python 2.6.

As Cura otherwise works in the still widely used Python 2.6 it seems best
to maintain this compatibility.