grass.sh: enforce Python 3 (#345)

Updates #289

lib/init/grass.sh

@@ -13,7 +13,7 @@
 #               setting the GISDBASE, LOCATION, and/or MAPSET.
 #               Finally it starts GRASS with the appropriate user
 #   	    	interface and cleans up after it is finished.
-# COPYRIGHT:    (C) 2000-2010 by the GRASS Development Team
+# COPYRIGHT:    (C) 2000-2020 by the GRASS Development Team
 #
 #               This program is free software under the GNU General
 #   	    	Public License (>=v2). Read the file COPYING that
@@ -24,7 +24,7 @@
 trap "echo 'User break!' ; exit" 2 3 9 15
 
 if [ -z "$GRASS_PYTHON" ] ; then
-    GRASS_PYTHON=python
+    GRASS_PYTHON=python3
 fi
 export GRASS_PYTHON