[PATCH evolve-ext v2] py3: broad pass for python3 compatibility

[Ludovic Chabant]

> I'm curious to hear what Ludovic thinks, but I'm doubtful that saves
> other contributors more time than it costs for Ludovic, so it's a net
> negative.
>
> I assume you know that Python 2's end of life is Jan 1 2020, so this
> is getting urgent. I don't know about others, but my team will need to
> have Mercurial and extensions on Python 3 by then.

At this point it's not too costly for me to update the patch (although
it's a bit annoying), especially since I'm not even running any tests
yet at this point: I just need a py3 mercurial that doesn't crash when
evolve is enabled, and seems to run ok for pull/push.

However, I would indeed prefer if someone from Octobus was to take it
away from me and deal with it ASAP. It's blocking several new features
for sourcehut, and, indeed, py3 EOL is approaching fast and both Drew
(lead dev on sourcehut) and Alpine (the distro on which sourcehut runs)
have aggressive timelines to completely remove any trace of py2 before
that date.

Plus, if someone from Octobus was to take this, it could also be part of
a broader "kickstart py3 support" task that would include setting up py3
builds on the CI, and other related tasks.

Cheers!