I agree that 3.5 is ancient and the focus should be to fix the newer versions of Python.

But given that the problem seems to be hard to reproduce -- I have the reproducer script from the original report running under the tracer since over a week now without a hit -- it seems to be beneficial to me to understand the issue on 3.5 to then check if 3.8+ is also affected.

Also note that Lib/mimetypes.py didn't change between 3.5.3 and 3.5.9.
The difference between 3.5.x and 3.6.x in this file doesn't affect the code flow, in fact the first commit that has a change to actually change the behavior I saw between 3.5.0 and current master is bpo-4963 that only went into 3.9.x.

I added dhess and steve.dower who were involved in bpo-4963 to the nosy list, maybe one of them remembers or is able to quickly spot the problem.