Load python3 before python2

[patrick96]

The old way causes issues with plugins that only support python3

Just calling has('python') will make vim load python2 (if it hasn't already loaded a python version).

So if the first thing you do when opening vim is call CtrlP, vim loads python2 and any plugin that only uses python3 won't work any longer.

Since this plugin supports python3 and we should use it whenever possible the best way is to first check if python3 is available so that vim loads python3