Consider backporting PEP 615

[systemcatch]

Just to make clear, backports.zoneinfo already exists and in Python 3.6+, I (as the maintainer of dateutil and the creator of PEP 615) highly recommend you use it whenever possible. The "backport" I'm referring to is that I'm planning to take the underlying logic from the pure-Python implementation of zoneinfo and move it into dateutil.tz, to the extent possible. That will fix a number of issues, but my major efforts to get PEP 615 ready and working in Python 3.9 made my backlog of work grow enormously ☹.

Originally posted by @pganssle in #802 (comment)