tests: convert ParseError arguments to str on Python 3
Arguments internally are bytes. Printing the exception on Python 3
will include b'' prefixes. This test file uses a .out file, which
doesn't support conditional output. The easiest way to get this to
pass on Python 3 is to normalize the exception before printing so
there are no b'' prefixes.
Differential Revision: https://phab.mercurial-scm.org/D5793