py3: convert _origdoc to sysstr to match __doc__
It's currently stored as bytes by core, so we need to convert it to
match Python's expected type for __doc__. This patch can be dropped if
D6623 gets accepted.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null