Added missing imports for test

IPython/core/tests/test_displayhook.py

@@ -1,4 +1,7 b''
+import sys
 from IPython.testing.tools import AssertPrints, AssertNotPrints
+from IPython.core.displayhook import CapturingDisplayHook
+from IPython.utils.capture import CapturedIO
 
 ip = get_ipython()