tests: allow xrange warning from perf.py
The use of xrange in this file is acceptable.
We need to make the output optional - and not conditional on
the Python version - because `pyflakes` could be executed by any
Python version and pyflakes behaves differently depending on the
Python version.
Differential Revision: https://phab.mercurial-scm.org/D6990
--- a/tests/test-check-pyflakes.t Sun Oct 06 09:27:10 2019 -0400
+++ b/tests/test-check-pyflakes.t Sat Oct 05 16:57:45 2019 -0400
@@ -21,4 +21,5 @@
> -X mercurial/thirdparty/zope \
> 2>/dev/null \
> | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+ contrib/perf.py:*: undefined name 'xrange' (glob) (?)