PEP 11 -- friendlier policy for unsupported platforms

[encukou]

Hello,
I'd like to propose updating PEP 11 to clarify unsupported platforms. Brett's summary of the proposal, with my addition:

• 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

The proposal: https://github.com/encukou/peps/pull/3/files
Discussion: https://discuss.python.org/t/proposed-pep-11-update-policy-for-unsupported-platforms/104986

[gpshead]

perhaps related to of #226 as well.