contrib: require Python 3.6 for byteify-strings.py

This script makes use of `token.COMMENT`, which apparently
isn't present until Python 3.6. So make the script and its
test conditional on Python 3.6.

contrib/byteify-strings.py

@@ -339,7 +339,7 def main():
 
 
 if __name__ == '__main__':
-    if sys.version_info.major < 3:
-        print('This script must be run under Python 3.')
+    if sys.version_info[0:2] < (3, 6):
+        print('This script must be run under Python 3.6+')
         sys.exit(3)
     main()

tests/test-byteify-strings.t

@@ -1,4 +1,4
-#require py3
+#require py36
 
   $ byteify_strings () {
   >   $PYTHON "$TESTDIR/../contrib/byteify-strings.py" "$@"