bug#70722: [PATCH] ; Migrate to python3 for (package-test-update-archives-async)

[Eli Zaretskii <eliz@gnu.org>]

> From: Lin Sun <sunlin7.mail@gmail.com>
> Date: Thu, 2 May 2024 18:11:04 +0000
> 
> The test case "package-test-update-archives-async" still tries to
> search "python2" for testing, while python2 was at the end of its life
> years ago.
> So move the test case from python2 to python3.

I'd prefer to make the test support both Python 2.x and Python 3.x.
The fact that python.org end-of-life'd Python 2 doesn't mean we have
to jump to attention and follow suit.  Old systems might still have
Python 2 installed, and there's no need to drop them.

I do agree that we should try looking for Python 3 first, and only
afterwards for Python 2.

Also, I think the test should start by looking for just "python",
before the versioned variants.

E.g., on my system (executable-find "python3") returns nil, although I
have Python 3.x installed -- but it's installed under the name
"python".

Thanks.