py3: simply use b'%d\n' to format pid in server.py Spotted by Martin, thanks.

py3: implement __bytes__() on most of our exception classes We store bytes in exc.args, which should be translated to a byte string without encode/decode dance. IOError subclasses are unchanged for now. We'll need to decide how our IOErrors should be caught.

py3: convert __doc__ back to bytes in help.py pycompat.getdoc() is pretty simple, but we wouldn't want to write handling of None inline.