Bail out if the version of requests is too old

### Problem

Debian stable (and perhaps others) have a version of requests that is far too old. We should bail out if we detect such a version.

Debian stable ships with 2.12.2, and the newest in pip is 2.4.2.

I still don't know what the earliest version that supports all of what we need is though.

The issue names no files, tests, or entry points. Start by locating dependency and startup version handling, then determine the earliest requests version that supports the project’s needs. Done means unsupported old installations fail clearly while supported versions continue to work.

Written by the indexing model from the issue text.