tests: make test-manifest.py portable to Python 3
Lots of b prefixes here, and https://bugs.python.org/
issue29714 means
that this test is still very broken on Python 3.6 and 3.6.1, but 3.6.2
should things (based on testing using tip of the 3.6 branch from git).
#cleanup-only
$ 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