Drop support for Python 3.4; add support for 3.8.

[icemac]

No description provided.

[jamadden]

Please don't drop support for Python 2 just yet.

[icemac]

@jamadden Is this package really used outside Zope 4?
Zope 5 is going to drop Python 2 support, if this package is not used outside I see no need to keep supporting Python 2. Zope 4 can use the previous version which still supports Python 2 if needed to run on Python 2.

[jamadden]

Yes this package is used outside Zope 4. ExtensionClass is used by Acquisition, and Acquisition is usedful and used in other contexts, including generic Zope 3 applications.

[icemac]

@jamadden What's your suggested schedule to remove Python 2 support in Zopefoundation packages?

[jamadden]

What's your suggested schedule to remove Python 2 support in Zopefoundation packages?

Basically, when it becomes otherwise completely untenable due to overall ecosystem changes. That's probably anywhere from six months to a year or two? It might depend on the project.

Most of these packages are core foundation packages that aren't going to see, at this point, a tremendous benefit from dropping Python 2 (especially as they'll continue to support Python 3.5). So I feel that they shouldn't be the forcing function; rather, they should continue to tick along quietly as they have been so that as people eventually find their own other forcing functions, they can make the switch in their own time, knowing that the core foundation packages that they've been using are both current and compatible and won't surprise them.

That is, if you've been running pip install -U … but have a bunch of dependencies that have python_requires=">2.7" and so never get upgraded, running the same on Python 3 shouldn't pull in a bunch of these low-level dependencies that have drifted from what you've been using in incompatible ways that are hard to diagnose. Lets help people keep everything as up-to-date as they can while they continue their own migrations. Make the flag day as painless as possible.

Let the "leaf" dependencies, the single-purpose libraries, the things that aren't deeply integrated into the core architecture of systems go first. Most zopefoundation projects should be the last things to drop support.

[icemac]

@jamadden I got your point. So let's postpone dropping Python 2 support for non-leaf libraries a some months. I updated this PR accordingly.

[jamadden]

LGTM!