Motivation: what do we mean about avoiding breakage for Py2? #2

As it stands, I think many people skimming the abstract will say "Releasing Py3-only packages breaks Py2? Well, duh!"

We need to make sure it's clear what we mean, because obviously code that requires Python 3 can't work on Python 2. Instead of talking about 'not breaking' Python 2, we need to say something like "ensuring that users still on Python 2 can easily install an older version, or a parallel 'LTS' version, which will still work for them".

Then there are a few strategies we can talk about:

1. Insist on Py2 users running pip install foo<5
2. Use a new package name, so Py3 users run pip install foo5
3. The metapackage trick, so foo depends on particular versions of foo_real.
4. Using the new metadata and machinery we've worked on so none of 1-3 are necessary.