context: rename local 'attr' to 'attr_'
In the next diff we're going to import mercurial.thirdparty.attr, and pyflakes
complains about this if this rename isn't done.
Differential Revision: https://phab.mercurial-scm.org/D897
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state