use pymode even for python3

pymode works for python 3 (python-mode/python-mode#197).  So far, this update (allowing it when python3 is present) seems to work fine for me.

Author: toejough

.vimrc

@@ -603,7 +603,7 @@
 
     " PyMode {
         " Disable if python support not present
-        if !has('python')
+        if !has('python') && !has('python3')
             let g:pymode = 0
         endif