The root issue (or at least one level closer to the root issue, that seems to be causing the decorator failure) is that the lower-constraints are not actually being enforced. Even though the logs should it is passing "-c [path to lower-constraints.txt]". So even though things should be constrained to a lower version, presumably a version that works with a different version of decorator, pip is still installing a newer package than what the constraints should allow.
There was a pip release on the 28th. Things don't look like they started failing until the 31st for us though, so either that is not it, or there was just a delay before our nodes started picking up the newer version.
I tested locally, and at least with version 19.3.1, I am getting the correctly constrained packages installed.
Still looking, but thought I would share in case that info triggers any ideas for anyone else.
I upgraded my pip and rebuilt the venv. The new pip has some good warnings emitted about some incompatible conflicts, so that part is good, but it did not change the package installation behavior. I am still able to get the correctly constrained packages installed locally on Fedora 32. So at least so far, it doesn't appear to be a pip issue.