Add __all__ to html5lib.treewalkers._base

Author: gsnedders

html5lib/treewalkers/_base.py

@@ -1,6 +1,9 @@
 from __future__ import absolute_import, division, unicode_literals
 from six import text_type, string_types
 
+__all__ = ["DOCUMENT", "DOCTYPE", "TEXT", "ELEMENT", "COMMENT", "ENTITY", "UNKNOWN",
+           "TreeWalker", "NonRecursiveTreeWalker"]
+
 import gettext
 _ = gettext.gettext