Add tiny amount of boilerplate to '+py3'

bin/mkx

@@ -88,8 +88,25 @@ mktemplate() {
         py3|Python3)
             echo "#!/usr/bin/python3"
             echo ''
-            echo "if __name__ == '__main__':"
-            echo '    '
+            echo 'import sys'
+            echo ''
+            echo ''
+            echo 'class UsageError(ValueError):'
+            echo '    pass'
+            echo ''
+            echo ''
+            echo 'def usage():'
+            echo '    self = sys.argv[0]'
+            echo '    raise UsageError("usage: %s [-j|-t|-p] YAML_FILE.." % self)'
+            echo ''
+            echo ''
+            echo 'def main(argv):'
+            echo '    if len(argv) < 2:'
+            echo '        usage()'
+            echo ''
+            echo ''
+            echo 'if __name__ == "__main__":'
+            echo '    main(sys.argv[1:])'
             ;;
 
         pya)