ensure deprecated_subcommands is defined

on the base Application class

cased AttributeError on subclasses other than TerminalIPythonApp

IPython/core/application.py

@@ -194,7 +194,9 b' class BaseIPythonApplication(Application):'
     #-------------------------------------------------------------------------
     # Various stages of Application creation
     #-------------------------------------------------------------------------
-
+    
+    deprecated_subcommands = {}
+    
     def initialize_subcommand(self, subc, argv=None):
         if subc in self.deprecated_subcommands:
             import time