5.2: move constraints generation code into release dir with tox

[mauritsvanrees]

• Move create-constraints.py out of the buildout config and into a new release directory. I am guessing here that no one actively uses the locally created constraints.txt. I could be wrong.
• Add release/tox.ini. You can call it with tox -c release/tox.ini -p auto. It takes care of a few things:
• Use a small Python 2 buildout to parse a release/constraints2.txt.
• Use a small Python 3 buildout to parse a release/constraints3.txt.
• Use a new script combine-constraints.txt to combine these two into release/constraints.txt fit for both Python versions, by adding modifiers like python_version < "3.0" where needed.

And then it does a few things that help me as release manager:

• Use pip2 download to download all packages in constraints.txt and put them in a release/dist directory. This only downloads Python 2 compatible package versions. Useful is that this complains and quits when a package is pinned to a version that is incompatible with Python 2. I updated a few version pins in versions.cfg for this.
• Use pip3 download to download all Python 3 compatible packages from constraints.txt.
• Copy versions.cfg and a few other files into the release/dist directory so I can scp this to dist.plone.org.

We could do this for Plone 6 as well, but there it is far less tricky, because everything is Python 3.

[jensens]

Like! And yes, for Plone 6 we need it too!