[PATCH STABLE] setup: tweak error message for Python 3

[Yuya Nishihara]

On Sun, 21 Apr 2019 08:57:12 -0700, Gregory Szorc wrote:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1555862221 25200
> #      Sun Apr 21 08:57:01 2019 -0700
> # Branch stable
> # Node ID 8211a51e17c85b80e39f68668798a91323242e9a
> # Parent  07faf5c65190f8fcc92c0a6663f8b9d223c4ebaa
> setup: tweak error message for Python 3

Queued for stable, thanks.