python: Add __bool__ helpers in addition to __nonzero__

Python 3 uses __bool__ instead of __nonzero__ when performing a
Boolean comparison.

Change-Id: I85185bbe136ecae67346fa23569e24edd7329222
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15996
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/python/m5/params.py

@@ -829,9 +829,12 @@ class Bool(ParamValue):
 
     # implement truth value testing for Bool parameters so that these params
     # evaluate correctly during the python configuration phase
-    def __nonzero__(self):
+    def __bool__(self):
         return bool(self.value)
 
+    # Python 2.7 uses __nonzero__ instead of __bool__
+    __nonzero__ = __bool__
+
     def ini_str(self):
         if self.value:
             return 'true'