Make on_setattr hooks accept generators

[vladfedoriuk]

Summary

Pull Request Checklist

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• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing-examples/baseline.py or, if necessary, typing-examples/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• The documentation has been updated.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
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    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 26.2.0, the next version is gonna be 26.3.0.
    If the next version is the first in the new year, it'll be 27.1.0.
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• Changes have news fragments in changelog.d.

[hynek]

as a quick preamble: there is no such thing as 25.5.0. The current version is 26.1.0 so the next is 26.2.0.