py3: replace file() with open() in test-import.t
file() is not present in Python 3.
This patch also adds a b'' prefix to make sure we write bytes in Python 3.
Differential Revision: https://phab.mercurial-scm.org/D2123
$ 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