arch: list.sort() sorting by key only in python3

list.sort() no longer accept a comparison function. Use the key argument
instead for key based sorting.

Change-Id: I2af2732628721faee3870af0d53c1267839896da
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26254
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

src/arch/isa_parser.py

@@ -1289,7 +1289,7 @@ class OperandList(object):
         return self.__internalConcatAttrs(attr_name, filter, [])
 
     def sort(self):
-        self.items.sort(lambda a, b: a.sort_pri - b.sort_pri)
+        self.items.sort(key=lambda a: a.sort_pri)
 
 class SubOperandList(OperandList):
     '''Find all the operands in the given code block.  Returns an operand