Switch to Python 3 as default

Discourage the usage of Python 2 by using Python 3 as default Python
everywhere.

src/xdgmenumaker

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # coding: utf-8
 # vim:et:sta:sts=4:sw=4:ts=8:tw=79:
 

tests/Makefile

@@ -1,5 +1,5 @@
 MM ?= ../src/xdgmenumaker
-PYTHON ?= python
+PYTHON ?= python3
 
 test: clean generate
 	@for i in `ls menus_correct/`; do \