PEP 598: CI implications, provisional APIs, other tweaks

[ncoghlan]

• explicitly discuss potential implications for CI and
  deployment management follow a useful discussion
  with Barry Warsaw
• propose locking down even provisional APIs once
  a given release series is feature complete
• note that delay in adopting major feature releases
  may prevent PEP 596 from having the desired
  effect of reducing feature delivery latency
• rephrase the section about Python 3.10 vs 4.0
  to make it clear that the PEP isn't proposing
  releasing 4.0 after 3.9, just discussing the impact
  the different choice of version number would have
  on the proposal
• reference Anthony Sottile's "Python 3.10" compatibility
  testing project