The SR of erp5-zope2 had the version of zc.buildout pinned to an
unpatched version that did not support build dependencies (
"setup-eggs").
This caused problems when trying to compile erp5-zope2 with recent
SlapOS in the test nodes, as it tries to use "setup_requires" instead,
which is not allowed:

```
Getting distribution for 'wstools'.
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
    ERROR: Command errored out with exit status 1:
     command: /srv/slapgrid/slappart19/t/eoa/shared/python2.7/3be205495452cf4067d1b6b952432c55/bin/python2.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-req-build-V_C9uf/setup.py'"'"'; __file__='"'"'/tmp/pip-req-build-V_C9uf/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-2KP9FV
         cwd: /tmp/pip-req-build-V_C9uf/
    Complete output (25 lines):
    DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
    WARNING: Url 'file:///dev/null/pytest-runner/' is ignored: it is neither a file nor a directory.
    ERROR: Could not find a version that satisfies the requirement pytest-runner (from versions: none)
    ERROR: No matching distribution found for pytest-runner
```

We remove these pins to use the latter version of buildout from
slapos.cfg.

In addition we adapt the erp5-zope2 buildout.cfg to buildout 3,
following the same steps as in 8c45efb1.