Better Plone 4 / Python 2 compatibility

[thet]

When I‌ tried to install this package in Plone 4 it took me some time to get the versions straight. It would have saved me some time if I had read the Plone 4 install section in the README, but I didn't and it is well hidden.

I think having an easy installation experience in Plone 4 with Python 2 is quite important for this package as this is probably a frequent use case.

This PR should fix that out of the box and improve the README a bit, as of the time of this writing. There is a good chance that it will break again, however, for now it does work fine.

/cc @canepan

[davisagli]

It looks to me like you're adding pins for Plone 4, but doing it based on the presence of Python 2. It is also possible to use Python 2 with Plone 5.1 and 5.2

[thet]

I‌ tested only in Plone 4.
But above these thresholds there is only Python 3 support - also for plone.schema, plone.restapi and plone.api.
I think this should be safe.

[davisagli]

@thet I think you are right.

[thet]

I‌ take that as a go :D
Time to merge, I've tested in in Plone 4 and Plone 6 and it works just fine.

[pbauer]

@thet weird, since this merge this packages main page does not render anymore in github, same for forks.
Also maybe related: @flipmcf ran into this issue during installation:

mr.developer: Updated 'collective.exportimport' with git.
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/home/mcfaddenm/repos/plone5.2_clean/rfasite/rfasite/eggs/mr.developer-2.0.1-py3.9.egg/mr/developer/common.py", line 181, in worker
    for lvl, msg in wc._output:
ValueError: too many values to unpack (expected 2)

[pbauer]

The issue has gone away :)

[thet]

Wow, great :)
Both issues?

@flipmcf issue looks like if you got a Python 2 incompatible package, actually the kind of problem my PRs here should have fixed.
But there will be similar problem popping up as soon as another dependency is released as Python 3 only package where we do not have any requirement constraints.

The other issue - I updated the README - maybe I created an .rst error.