[3.13] gh-113433: Automatically Clean Up Subinterpreters in Py_Finalize() (gh-121067)

This change makes things a little less painful for some users.  It also fixes a failing assert (gh-120765), by making sure all subinterpreters are destroyed before the main interpreter.  As part of that, we make sure Py_Finalize() always runs with the main interpreter active.

(cherry picked from commit 4be1f37b20bd51498d3adf8ad603095c0f38d6e5, AKA gh-121060)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>