gdb-gdb.py.in: Fix ordering of TypeFlags objects with Python 3

Python 3 doesn't use __cmp__ to order objects, it uses __lt__.  Because
of this, we get this exception when trying to pretty-print "type"
objects:

I tested it with Python 2.7 as well.

gdb/ChangeLog:

	* gdb-gdb.py.in (TypeFlag) <__cmp__>: Remove.
	<__lt__>: Add.

gdb/gdb-gdb.py.in

@@ -38,9 +38,11 @@ class TypeFlag:
         self.name = name
         self.value = value
         self.short_name = name.replace("TYPE_INSTANCE_FLAG_", '')
-    def __cmp__(self, other):
+
+    def __lt__(self, other):
         """Sort by value order."""
-        return self.value.__cmp__(other.value)
+        return self.value < other.value
+        
 
 # A list of all existing TYPE_INSTANCE_FLAGS_* enumerations,
 # stored as TypeFlags objects.  Lazy-initialized.