Drop support for Python 2.7 and 3.5.

[icemac]

No description provided.

[dataflake]

.travis.yml and setup.py still refer to Python 2

[mgedmin]

lgtm, assuming we're fine with dropping 2.7 as a group

(I wanted to link to a previous discussion about that but couldn't find it)

[dataflake]

lgtm, assuming we're fine with dropping 2.7 as a group

Michael worked on several packages yesterday and wanted to just drop 3.5 first, because Zope 5 doesn't support it. But he overlooked that dropping 3.5 would make a package potentially incompatible with Zope 4.x, which explicitly supports it. Then we both reasoned that he could just make the full step of dropping 2.7 as well, so the package would end up for Zope 5 only. But Jason Madden spoke up about some packages where he definitely wants to retain 2.7.

I'm now thinking that we should stop such efforts to drop compatibility unless there are really good reasons and revisit it when Zope 5 has been around a bit and adopted by the bigger "customers" in the ecosystem, e.g. Plone.

[dataflake]

To get this out of the critical path for a Zope 5 release I have pulled out the changes that add support for Python 3.8 and Python 3.9 and merged those into master. I then published release 4.2, which we can use for Zope 5 (and 4).

This PR only contains the changes for dropping Python 2 and Python 3.5 support, and that should wait until we have consensus on when to drop this support for packages where keeping that support doesn't introduce a big burden. I added the do not merge tag for that reason.

[dataflake]

Superceded by #14