contrib: require Python 3.7 for byteify-strings.py stable

contrib: require Python 3.7 for byteify-strings.py

bb509f39d387 made an error, it's actually 3.7 that introduced token.COMMENT.

Differential Revision: https://phab.mercurial-scm.org/D7220

--- a/contrib/byteify-strings.py	Mon Nov 04 12:20:11 2019 -0500
+++ b/contrib/byteify-strings.py	Mon Nov 04 19:19:36 2019 +0000
@@ -339,7 +339,7 @@
 
 
 if __name__ == '__main__':
-    if sys.version_info[0:2] < (3, 6):
-        print('This script must be run under Python 3.6+')
+    if sys.version_info[0:2] < (3, 7):
+        print('This script must be run under Python 3.7+')
         sys.exit(3)
     main()

--- a/tests/test-byteify-strings.t	Mon Nov 04 12:20:11 2019 -0500
+++ b/tests/test-byteify-strings.t	Mon Nov 04 19:19:36 2019 +0000
@@ -1,4 +1,4 @@
-#require py36
+#require py37
 
   $ byteify_strings () {
   >   $PYTHON "$TESTDIR/../contrib/byteify-strings.py" "$@"