drop six and python2 support

ox/js.py

@@ -2,19 +2,12 @@
 # -*- coding: utf-8 -*-
 # vi:si:et:sw=4:sts=4:ts=4
 
-from six import PY2
 from .utils import json
 
 def minify(source, comment=''):
     # see https://github.com/douglascrockford/JSMin/blob/master/README
     def get_next_non_whitespace_token():
         pass
-    # python2 performance with unicode string is terrible
-    if PY2:
-        if isinstance(source, unicode):  # pylint: disable=undefined-variable
-            source = source.encode('utf-8')
-        if isinstance(comment, unicode):  # pylint: disable=undefined-variable
-            comment = comment.encode('utf-8')
     tokens = tokenize(source)
     length = len(tokens)
     minified = '/*' + comment + '*/' if comment else ''