need a "lower constraints" CI job

[Brian Rosmaita]

We've discontinued maintaining the lower-constraints file, but we still have the following issue:

1. our (test-)requirements.txt files specify the minimum version of dependencies
2. the actual versions used for testing tend to be closer to whatever is in upper-constraints.txt in the requirements repository
3. it's possible for us to actually need a higher version of some dependency than is specified in (test-)requirements.txt

We don't notice #3 until someone takes our specified minima seriously and reports a problem.

I've got a DNM patch that I hacked together to check the minima before releasing from master. Should harden that a bit and set up a periodic CI job to run it in master. Will probably have to be a regular job on the stable branches to avoid issues like Bug #1948934.