PEP 750: restrict support for Template + str addition

[davepeck]

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

Following this PR opened with the steering-council and related discussion, we propose an update to PEP 750 that:

• Allows Template/Template concatenation (both implicit and explicit)
• Disallows any form of Template/str concatenation (both implicit and explicit)

A rationale is provided and the two relevant sections of the PEP are updated ("Template String Concatenation" and, in rejected ideas, "Disallowing Template Concatenation").

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📚 Documentation preview 📚: https://pep-previews--4395.org.readthedocs.build/

[AA-Turner]

Adding the label to prevent inadvertent merging before pronouncement

[davepeck]

I've updated this PR to match the steering council decision on template concatenation. /cc @AA-Turner