css: explicitly use Python 3.

diff --git a/css/postprocess.py b/css/postprocess.py
index 787db97ecfb72ae1d5d3a86a4fc9aaf218d47c28..a62e998be911a3ca0db8c94b7767caa3a63cfebf 100755 (executable)
--- a/css/postprocess.py
+++ b/css/postprocess.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 #
 #   This file is part of m.css.