Serhiy, about your 'huge patch' to modernize code:

I am more positive than some because:

1) To me, a one-time gentile change is not 'churning'.

2) As we link to many, most, or even all python-coded stdlib modules (I think there is a proposal for 'all'), there is more benefit to using modern idioms.

On the other hand, 'huge' patches can be too much to discuss, justify, and review all at once.

Using {.. } for sets consistently is a nice-sized chunk to consider.  We can identify, discuss, and decide on each sub-case (I have identified 4 so far).  It has the additional benefit of being a performance enhancement.
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'set((...' is used in distutils (which I will not change) and in many tests.  So that is not an issue.  'frozenset((' is used 5 times in regular module code.