.arcconfig

setup: only allow Python 3 from a source checkout (issue5804)

People are running `pip install Mercurial` with Python 3 and that
is working because not everything performs a Python version
compatibility check.

Modern versions of pip do recognize the "python_requires" keyword
(https://packaging.python.org/tutorials/distributing-packages/#python-requires)
which we set if using setuptools. But this isn't set nor recognized
everywhere.

To prevent people from accidentally installing Mercurial with Python
3 until Python 3 is officially supported, have setup.py fail when
run with Python 3. But don't fail if we're running from a source
checkout, as we don't want to anger Mercurial developers hacking
on Python 3 nor Mercurial's test automation running from source
checkouts. People running setup.py from source checkouts could still
fall through a Python 3 crack. But at least the
`pip install Mercurial` attempt will get nipped in the bud.

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