bpo-44441: _PyImport_Fini2() resets PyImport_Inittab (GH-26874) (GH-26877)

Py_RunMain() now resets PyImport_Inittab to its initial value at
exit. It must be possible to call PyImport_AppendInittab() or
PyImport_ExtendInittab() at each Python initialization.

(cherry picked from commit 489699ca05bed5cfd10e847d8580840812b476cd)