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This is an implementation along the lines of what I proposed in #754. One difference from that is I moved errors that occur in all of py35-py38 into a shared whitelist. This felt like a good balance between reducing redundancy, preserving the ability to easily detect unused whitelist entries and implementation simplicity. Another difference is that I incorporated the discussion from #3693.
This is a draft until python/mypy#8325 gets merged (cc @JelleZijlstra ), at which point I can switch things to use mypy instead of my fork, as noted in the last commit.
Errors have come down substantially: overall by about half, py38-only by about 70%, as measured by the decrease since I first posted in #754.
Also happy to split out the README changes if that's better.