remove unnecessary try-excepts

we only import from _decorators when importing from
numpy.testing.decorators failed, so there is no point in trying to do
it again.

IPython/external/decorators/__init__.py

@@ -3,5 +3,4 b' try:'
     from numpy.testing.noseclasses import KnownFailure
 except ImportError:
     from _decorators import *
-    # KnownFailure imported in _decorators from local version of
-    # noseclasses which is in _numpy_testing_noseclasses
+    from _numpy_testing_noseclasses import KnownFailure

IPython/external/decorators/_decorators.py

@@ -20,15 +20,8 b' import warnings'
 #from numpy.testing.utils import \
 #        WarningManager, WarningMessage
 # Our version:
-try:
-    from numpy.testing.utils import WarningManager, WarningMessage
-except ImportError:
-    from _numpy_testing_utils import WarningManager, WarningMessage
-
-try:
-    from numpy.testing.noseclasses import KnownFailure, KnownFailureTest
-except ImportError:
-    from _numpy_testing_noseclasses import KnownFailure, KnownFailureTest
+from _numpy_testing_utils import WarningManager
+from _numpy_testing_noseclasses import KnownFailureTest
 
 # End IPython changes