Fix missing sys import (#239)

Author: cclauss

stacks_queues/stack_min/test_stack_min.py

@@ -1,3 +1,4 @@
+import sys
 from nose.tools import assert_equal
 
 
@@ -41,4 +42,4 @@ def main():
 
 
 if __name__ == '__main__':
-    main()
+    main()