Import sys before using it in warnings.

theano/tensor/signal/downsample.py

@@ -11,6 +11,7 @@ import numpy, theano
 import __builtin__
 
 def max_pool2D(*args, **kwargs):
+    import sys
     print >> sys.stderr, "DEPRECATION: max_pool2D renamed to max_pool_2d"
     return max_pool_2d(*args, **kwargs)