PEP 667: Clarify specification ambiguities and the expected impact (python#3845)

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Co-authored-by: Carl Meyer <[email protected]>
Co-authored-by: Carol Willing <[email protected]>
Co-authored-by: Alyssa Coghlan <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>
Co-authored-by: Mark Shannon <[email protected]>

Author: 6 people