.arcconfig

pycompat: explicitly prefix builtin attr usage with `builtins.`

It doesn't seem like this would fix any bug, because the wrapped functions that
take bytes instead of str are defined after these calls.  But PyCharm was
flagging the second and third uses, saying "Type 'str' doesn't have expected
attribute 'decode'".  It wasn't flagging the first, but I changed it for
consistency.

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