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

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

Pick up nexedi/wendelin.core@3d0f134c to
fix build on Python3.