tests: allow warning about file

The code is protected by a block that makes it Python 2 only, so
use of file is acceptable.

Differential Revision: https://phab.mercurial-scm.org/D7018

tests/test-check-pyflakes.t

@@ -23,4 +23,5 b' run pyflakes on all tracked files ending'
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   contrib/perf.py:*: undefined name 'xrange' (glob) (?)
   mercurial/hgweb/server.py:*: undefined name 'reload' (glob) (?)
+  mercurial/util.py:*: undefined name 'file' (glob) (?)