[From the cited python-dev email]:

"Python 3.7 and master (3.8) are affected. As of now, both branches use
OpenSSL 1.1.0 and must be updated to 1.1.1 soonish. Ned has scheduled
3.7.3 release for 2019-03-25. That's still well within the release
schedule for 1.1.0. I suggest that we update to 1.1.1 directly after the
release of Python 3.7.3 and target 3.7.4 as first builds with TLS 1.3
support. That gives Victor, Steve, and me enough time to sort out the
remaining issues."

So setting the priority here to "deferred blocker" as a reminder to take care of this prior to 3.8.0b1 (2019-05-26) and 3.7.4rc1 (2019-06-10) at the latest.