Adapt SRs to use zc.buildout 3.0.1+slapos002.

Gotchas and relevant details:

• During slapos.rebootstrap, zc.buildout and its dependencies are reinstalled using the same egg versions, just another Python version. Thus SRs which install Python2 need to use Python2-compatible versions all along. For this stack/slapos-py2.cfg was introduced to use Python2-compatible versions of setuptools, pip and wheel.

• zc.buildout 3.0.1+slapos002 supports pyproject.toml and should thus reduce the need for using wheels by pinning versions with :whl. However pyproject.toml introduces a multitude of alternative build backends (where before it was always setuptools), which means those build-backends have to be installed too, and provided as setup-eggs to the recipe installing the eggs which need these build-backends. For expediency, a lot of eggs where pinned with :whl, along with a commentary of the build-backend which would be needed to install without :whl. A further complicating factor is that some build-backends require themselves as build-backend (e.g. poetry), thus for such build-backends using :whl will be the most practical solution anyway.