[PATCH STABLE] setup: only allow Python 3 from a source checkout (issue5804)

[Yuya Nishihara]

On Fri, 23 Feb 2018 18:02:20 -0800, Gregory Szorc wrote:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1519437424 28800
> #      Fri Feb 23 17:57:04 2018 -0800
> # Branch stable
> # Node ID 5b80235fd920c02ff430d8b0989fc55a58f11a0c
> # Parent  5da7b8cb6f751fa7a331ed501d26f336e1bbc8f9
> setup: only allow Python 3 from a source checkout (issue5804)

Queued for stable, thanks.