PEP 466: further simplify the proposal

- dropped the "legacy ssl" branch idea again
- explained why selective backports are a bad idea
- better explained how long term support differs from short term
- better explained some downstream constraints
- explained that downstream redistributors have been respecting
  our upstream policies, potentially against their own better
  judgement
- used Red Hat's support cycle as a reference model
- removed the snarky comments that hit unintended targets
- note the need for Windows specific contributions as an open question
- not the general need for contributions of time as an open question
- last open question is whether or not we're happy this is the best
  available solution given downstream constraints
incorporated %a comments; general clean-up