Not too infrequently, people ask questions like we can use C++17 or some feature in a newer version of Python. We have a rough methodology for addressing these questions, but it's not documented anywhere and it might mean different things to different people.

Writing this down serves several purposes:

1. it forces us to get on the same page about the policy
2. it is handy to point people to when the question inevitably recurs
3. it aids in planning: looking at the document we ought to be able to identify a rough date when some feature or library can be taken advantage of. That can assist in evaluating tradeoffs, timing prep work to take advantage of a particular feature, or deciding whether we want to live with a suboptimal situation that can be resolved by a future upgrade.

cc @brianjo @mruberry