zope4py3 commits worth merging on their own

With @jerome, we talked about how to merge zope4py3 branch to master and we think the best solution is to take the whole diff and make one or several big commits. I checked the whole zope4py3 branch history and I think that some commits should be committed directly as they are (either because the message is atomic or because it is not a bytes <-> str-like commit), hence this MR.

For each commit, I tried to pick up relevant changes in other WIP commits (and for some squashed them).

There is only round2-related change that is not in this branch because it's not thread-safe so maybe we can discuss about it here:

• patches/python: workaround safeimage impacted by this round2 patch
• patches/python: override py3 builtin round to keep py2 behavior

Cc'ing @kazuhiko and @vnmabus