Until now it wasn't but with python3, import on future (not part of Python
library) means ${buildout:executable} is not enough. Anyhow, genbt5list should
be executed with all eggs IMO to avoid such problem.

I don't see any reason why we don't use upstream zope *versions.cfg files. This
just duplicates work and makes maintenance tedious especially now that we have
zope2, zope4py2 and zope4py3.

Also:
  * Remove pinned versions on eggs with newer version or already defined in
    Zope versions.cfg files.
  * Make sure that pinned versions in stack/slapos.cfg do not override Zope versions
    (for example ZConfig versions set in stack/slapos.cfg was creating a version
    conflict while installing another egg).

SIGUSR2 for reopening log files was handled by ZServer and waitress does not
handle signals at all, so we could handle this signal but Zope official
documentation recommends using logrotate `copytruncate`:
  https://zope.readthedocs.io/en/4.7/zopebook/MaintainingZope.html#log-rotation
  https://github.com/zopefoundation/Zope/issues/809

This fixes failures in erp5_dms:testWebDavSupport.

  Traceback (innermost last):
    [...]
    Module Products.ERP5Type.patches.OFSImage, line 94, in PUT
      return PUT_orig(self, REQUEST, RESPONSE)
    Module OFS.Image, line 665, in PUT
      data = data.encode('UTF-8')
  UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 10: ordinal not in range(128)

Upgrading PyStemmer requires Cython.

Server header might be changed by client facing caddy, for which caddy code
has been fixed to not transmit the Server header at all by default.
In ATS Server header has been disabled by configuration.
haproxy does not add Server header at all.
Thanks to improvement in the test backend, it's asserted that Server header
send by the backend is left intact.

This was useless. dumps is enough.

And upgrading to the python3-supported version (1.1) requires gtk3 and recent
versions of freetype, glib, atk, gtk, gtkmm, atkmm...

https://wiki.inkscape.org/wiki/Tracking_Dependencies

Besides of consistency, this also avoids zope eggs versions to be overriden elsewhere.

This reverts commit 27f574bc.

NEO now should support zodbpickle >= 2 with nexedi/neoppod!19.

/cc @jerome
/merge-request-on nexedi/slapos!1125