py3: use pycompat.bytestr to convert _b85chars to bytes The tranformer does append b'' to the value and make that a bytes but bytes in Python 3 returns the ascii value on getting characters using indexing. Characters of this string are queried using indexing multiple times in the file and to support that we use pycompat.bytestr which returns the bytechrs using indexing. Differential Revision: https://phab.mercurial-scm.org/D2072