Release 8.1

[cjwatson]

• I signed and returned the Zope Contributor Agreement, and received and accepted an invitation to join a team in the zopefoundation GitHub organization.
• I verified there aren't any other open pull requests for the same change.
• I followed the guidelines in Developer guidelines.
• I included a change log entry in my commits.

It's been a while since I released a Zope package, but I wanted to get the setuptools dependency removal out, so I ran fullrelease on it and then got a "Protected branch update failed" message near the end. Not sure if I did something wrong ...

[icemac]

@cjwatson You did everything right: Only very few people are allowed to push directly to master but many people are allowed to release. We did not find a solution yet besides this release PR afterwards.

[cjwatson]

The downside is that the tag has been left orphaned (not corresponding to anything on master). I guess I should have been more careful to mark this PR as "must not be squashed" somehow. Not the end of the world but a little confusing.

[icemac]

Right, I nowaday ususally squash merge without thinking but this could have been a normal merge. Maybe I'll keep this in mind for the future.