Add future imports to extras files

properties/extras/singleton.py

@@ -1,4 +1,9 @@
 """Singleton behavior for tracking individual objects (enum-like)"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 import six
 
 from ..base import HasProperties, PropertyMetaclass

properties/extras/uid.py

@@ -1,4 +1,9 @@
 """Classes for dealing with HasProperties instances with unique IDs"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 import uuid
 
 from six import string_types, text_type

properties/extras/web.py

@@ -1,4 +1,9 @@
 """Property classes for web-related concepts"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 from six.moves.urllib.parse import ParseResult, urlparse                       #pylint: disable=import-error
 
 from .. import basic