Thanks for identifying this issue and breaking it out into a separate bpo, Jason.  If I understand correctly, the problematic fix for Issue30458 has already been released in maintenance release 3.7.4 and security release 3.6.9, is in the current security release candidate 3.5.8rc1, as well as 3.8.0b4, and, without further action, will be in 2.7.17rc1 and continue to be in 3.7.5rc1.  In other words, this issue potentially affects all currently maintained Python branches and/or releases.  (In addition, there appear to be still unresolved questions about the original Issue30458 and the CVE's associated with it.  But let's ignore those here. My brain hurts enough already.)

The immediate question for me is what to do about 3.7.5.  We could:
1. hold 3.7.5rc1 for a mitigation fix
2. release 3.7.5rc1 and accept a fix for 3.7.5final or for an unplanned 3.7.5rc2
3. fix in 3.7.6
4. do nothing other than possibly a doc change

Since 3.5.8rc1 is already released for testing, a similar decision needs to be made for it.

And 3.8.0rc1 and 2.7.17rc1 are schedulded for tagging om the coming weeks.

Since the problem. as best I understand, is most likely to impact tests rather than legitimate user cases (is that correct?) and, since at least some projects and users of 3.7.4 impacted by the change have developed workarounds, and since 3.7.5rc1 is being delayed pending a resolution of this, I think the best options for 3.7.5 at this point are either 2 or 3 above.  So, unless someone expresses a major objection in the next few hours, I am going to proceed with 3.7.5rc1 as is with the hope that we will have final resolution prior to 3.7.5 final.

Decisions will still have to be made by the other RMs for their branches.