PEP 11: Friendlier policy for unsupported platforms

[encukou]

Update PEP 11 to clarify unsupported platforms.

• We have code for unsupported platforms and that’s OK, but it doesn’t imply any promises either.
• We welcome and encourage people that support CPython on other platforms – without promising any particular level of welcoming and encouragement.

Also:

• Add concrete rules for POSIX features (so we don't “over-fit” to Linux ∩ Mac ∩ BSD)
• Encourage listing “outsider” experts in the devguide, and pinging the experts

SC decision: encukou#3 (review)
Discussion: https://discuss.python.org/t/proposed-pep-11-update-policy-for-unsupported-platforms/104986

• 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)

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📚 Documentation preview 📚: https://pep-previews--4790.org.readthedocs.build/