.arcconfig

pure: use string for exception in the pure version of base85

Without this change, running the test with python3 and --pure gives the
following error::

  --- /home/marmoute/src/mercurial-dev/tests/test-import-git.t
  +++ /home/marmoute/src/mercurial-dev/tests/test-import-git.t.err
  @@ -518,7 +518,7 @@
     >
     > EOF
     applying patch from stdin
  -  abort: could not decode "binary2" binary patch: bad base85 character at position 6
  +  abort: could not decode "binary2" binary patch: b'bad base85 character at position 6'
     [255]

     $ hg revert -aq

To make the cext implementation, we use a "native" string for the exception.
This fix the test failure.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}