Incidentally, this also fixed the warning not showing up if a subinterpreter wasn't
cleaned up via _interpreters.destroy. I had to update some of the tests as a result.
This brings the code under test.support.interpreters, and the corresponding extension modules, in line with recent updates to PEP 734.
(Note: PEP 734 has not been accepted at this time. However, we are using an internal copy of the implementation in the test suite to exercise the existing subinterpreters feature.)
