compat: don't rely on cpython-specific builtins manipulation
Pierre Augier signaled on the mailing list that this fails on Pypy and pointed
out the correct solution.
Differential Revision: https://phab.mercurial-scm.org/D11732
$ 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