don't unconditionally import nose

Commit b64f754008964caef3b2a7620510b5c80f755fb0
broke IPython entirely in the absence of nose (even setup.py).

IPython/external/decorators/__init__.py

@@ -3,4 +3,7 b' try:'
     from numpy.testing.noseclasses import KnownFailure
 except ImportError:
     from _decorators import *
+    try:
     from _numpy_testing_noseclasses import KnownFailure
+    except ImportError:
+        pass

IPython/external/decorators/_decorators.py

@@ -21,7 +21,10 b' import warnings'
 #        WarningManager, WarningMessage
 # Our version:
 from _numpy_testing_utils import WarningManager
+try:
 from _numpy_testing_noseclasses import KnownFailureTest
+except:
+    pass
 
 # End IPython changes