Fix version pins for Python 3.7 and 3.8.

[icemac]

Now running all buildout sections in tox to prevent such problems in the future.

Open issues:

• Port handling of Python version dependent pins from Zope 4.x to util.py not needed it is already in place but only versions-prod.cfg is rendered into requirements.txt and constraints.txt.
• Python 3.7 seems to need a different version for zipp
• Windows seems to need colorama for all Python versions.