fix __doc__ assignment in Py3

tensorpack/train/base.py

@@ -4,6 +4,7 @@
 
 import time
 import weakref
+import six
 from six.moves import range
 
 import tensorflow as tf
@@ -293,6 +294,7 @@ def _get_property(name):
     """
     ret = property(
         lambda self: getattr(self.loop, name))
+    if six.PY3:     # __doc__ is readonly in Py2
         try:
             ret.__doc__ = getattr(TrainLoop, name).__doc__
         except AttributeError: