contrib: have byteify-strings explode if run in Python 2

Differential Revision: https://phab.mercurial-scm.org/D6341

contrib/byteify-strings.py

@@ -7,7 +7,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 
 import argparse
 import contextlib
@@ -227,4 +227,7 b' def main():'
                 process(fin, fout, opts)
 
 if __name__ == '__main__':
+    if sys.version_info.major < 3:
+        print('This script must be run under Python 3.')
+        sys.exit(3)
     main()