.arcconfig

dynamic-import: use sysstr for importing extension and others

This logic is used by extensions, and python hooks and merge-tools. All this
logic eventually deals with native string (unicode in Python 3). This patch
makes it handle `str` directly instead of relying on some pycompat low lever
layer to do the conversion at the last minutes.

We adjust the Python version filtering of a test as the output seems to be present with Python 3.7 too.

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