gh-145055: Accept frozendict for globals in exec() and eval()

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• Issue: The globals argument to eval() should accept frozendict #145055

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📚 Documentation preview 📚: https://cpython-previews--145072.org.readthedocs.build/

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The PR is now ready for review.

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cc @skirpichev

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It's still possible to modify builtins, unless builtins is declared as a frozendict:

# module __builtins__
>>> fd=frozendict(__builtins__=__builtins__)
>>> exec("__builtins__.x = 1", fd, fd)

# dict __builtins__
>>> fd=frozendict(__builtins__=__builtins__.__dict__)
>>> exec("__builtins__['x'] = 1", fd, fd)

# frozendict __builtins__
>>> fd=frozendict(__builtins__=frozendict(__builtins__.__dict__))
>>> exec("__builtins__['x'] = 1", fd, fd)
Traceback (most recent call last):
  ...
TypeError: 'frozendict' object does not support item assignment

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@skirpichev: I removed the sentence from the documentation, and I replaced it with a more precise error message if assigning __builtins__ to the frozendict globals fails. Please review the updated PR.

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Merged. Thanks for the review @skirpichev.