use py.builtin.callable instead of raw callable in _pytest/runner.py

This is consistent with how callable() is called from the rest of pytest code
(see _pytest/nose.py & _pytest/python.py) plus, as a nice side-effect, it
makes pytest work correctly on Python 3.1.

--HG--
branch : python 3.1 fix

_pytest/runner.py

@@ -328,7 +328,7 @@ class SetupState(object):
         is called at the end of teardown_all().
         """
         assert colitem and not isinstance(colitem, tuple)
-        assert callable(finalizer)
+        assert py.builtin.callable(finalizer)
         #assert colitem in self.stack  # some unit tests don't setup stack :/
         self._finalizers.setdefault(colitem, []).append(finalizer)