gh-76785: Update test.support.interpreters to Align With PEP 734 (gh-115566)

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.)