PEP 747: Rephrase rules for | operator. Remove ability to assign TypeExpr[X | Y | ...] to UnionType.

[davidfstr]

Also:

• Streamline compare/contrast of TypeExpr vs. TypeAlias

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• Change is either:
  • [x] To a Draft PEP
  • [ ] To an Accepted or Final PEP, with Steering Council approval
  • [ ] To fix an editorial issue (markup, typo, link, header, etc)
• [x] PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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