contrib/asv.conf.json

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.

{
    "version": 1,
    "project": "mercurial",
    "project_url": "https://mercurial-scm.org/",
    "repo": "..",
    "branches": ["default", "stable"],
    "environment_type": "virtualenv",
    "show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
    "benchmark_dir": "benchmarks",
    "env_dir": "../.asv/env",
    "results_dir": "../.asv/results",
    "html_dir": "../.asv/html"
}