Add pipenv caveat about mixing Python versions

[ncoghlan]

Due to pypa/pipenv#857, pipenv can currently get environment marker evaluation wrong when generating the lock file if the target venv uses a version of Python other than the one running pipenv.

This is most frequently a problem when setting up a Py2 venv from Py3 (due to the prevalence of Py2 only conditional dependencies), but issues can also arise when creating venvs for older Python 3 versions.

[theacodes]

should we just wait for the upstream issue to be addressed or do we anticipate this guide needing this caveat long-term?

[ncoghlan]

I think it's worth adding the caveat in the near term, as the pipenv folks have been trying different ways of fixing this over the past few releases, and it isn't clear yet when a more robust approach to mixing and matching Python versions will ship.

As things currently stand, we're seeing folks trying to use a recent Python 3 to manage Python 2.7 projects (rather than developing & deploying on the same Python version), and wondering why their Python-2-only dependencies aren't showing up properly (and getting an overly negative impression of pipenv as a result).

Even after it's fixed, we may want to keep the caveat in some form, just noting the minimum version needed for reliable cross-version operation.

[theacodes]

Sounds good. I'll try to get this up this week, but if someone wants to send a PR to beat me to it I'll merge quickly. :)