entrypoints=0.3 rquires flit as build backend

pkgconfig=1.5.1 requires poetry as build backend.

Earlier version 1.22.0 does not support recent setuptools.

Latest version 1.26.2 has more complex build requirements,
left for later.

Now that buildout uses pip to correctly follow pyproject.toml,
cattrs=22.2.0 requires poetry as the build backend.

For now it's easier to install as wheel than to install poetry and
then use it as setup-egg for cattrs. This must be done eventually.

zc.buildout=2.7.1+slapos020 is SlapOS-patched buildout2 with support
for bootstrapping buildout3, required to be able to install buildout3.

Downgrade related eggs to pre-buildout3 versions as well.

Version up zc.buildout, pip and zc.recipe.egg.

Version up slapos.rebootstrap to adapt to buildout3.

This is a follow-up of 7a1ba8df.

because re6stnet only supports OpenVPN 2.4.
This reverts commit 6802867d.

When compiling older GCC versions in newer machines, libsanitizer
does not compile, showing a error: `multiple definition of
'enum fsconfig_command'` (e.g.
https://github.com/llvm/llvm-project/issues/56421).
This is because starting from Glibc 2.36, it is not possible to include
both "linux/fs.h" and "sys/mount.h".

We can apply a patch, similar to the one in
https://reviews.llvm.org/D129471 , but with the extension and lines
changed to match the file. This is also similar to the upstream fix.

See merge request nexedi/slapos!1579

This will be used to add the sqlite3 binary directory to PATH.

See nexedi/neoppod@5923f8f5

This now supports producing html reports directly

nbd2-host and nbd2-port are asserted too.

Test are reorganized to easily assert mixed scenarios.

Zope no longer serve XML-RPC in ERP5, unless it is explicitly enabled
using this.

See nexedi/erp5!1931

--enable-readline is broken:
  - it searches for readline.h instead of readline/readline.h
  - it does not abort when it fails to find headers
Use --with-readline-inc so that it sets HAVE_READLINE.

Also use the dedicated configure option to enable update limit.
And sqlite3 does not depend directly on ncurses.

See nexedi/neoppod@71564067

See merge request nexedi/slapos!1566

See merge request nexedi/slapos!1576

See merge request nexedi/slapos!1572

to avoid the following error:
relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC

software/osie-coupler: keep the build source directory instead of having another open62541-source part.