util: use pycompat.bytestr in checkwinfilename

Fixes `hg add` on python3.

mercurial/util.py

@@ -1177,7 +1177,7 b' def checkwinfilename(path):'
     for n in path.replace('\\', '/').split('/'):
         if not n:
             continue
-        for c in n:
+        for c in pycompat.bytestr(n):
             if c in _winreservedchars:
                 return _("filename contains '%s', which is reserved "
                          "on Windows") % c