[$] Subinterpreters for Python

A project that has been floating around in the Python world for a number ofyears is now working its way toward inclusion into the language-or not."Subinterpreters", which are separate Python interpreters that cancurrently be created via the C API for extensions, are seen by some as away to get a more Go-like concurrency model for Python. The first steptoward that goal is to expose that API in the standard library. But thereare questions about whether subinterpreters are actually a desirablefeature for Python at all, as well as whether the hoped-for concurrencyimprovements will materialize.