Revert "Check __setattr__ when property is not settable (#9196)"

[hauntsaninja]

This reverts commit 3186281.

I realised that object in typeshed has __setattr__ defined. Currently it's a pretty common idiom for marking read-only attributes using properties, and since every class has a __setattr__ in its MRO, this breaks that.

E.g, this no longer produces an error:

from datetime import date
dt = date(2000, 1, 1)
dt.year = 2003

Additionally, it introduces a broken test on master, since we (unintentionally) don't run some Python2 tests in CI (filed as #9473 )