py3: use bytes literal in test-hgweb-json.t

# skip-blame just b'' prefix

Differential Revision: https://phab.mercurial-scm.org/D4970

tests/test-hgweb-json.t

@@ -2201,7 +2201,7 b" Commit message with Japanese Kanji 'Noh'"
 Commit message with null character
 
   $ echo foo >> da/foo
-  >>> open('msg', 'wb').write('commit with null character: \0\n') and None
+  >>> open('msg', 'wb').write(b'commit with null character: \0\n') and None
   $ hg ci -l msg
   $ rm msg