Py_NewInterpreter() (new_interpreter() in practice) and Py_InitializeEx() now share almost all their code.

Py_EndInterpreter() shares almost all its code with Py_Finalizer(). It's not perfect, but it's way better than previously.

Py_NewInterpreter() now isolates more things from the main interpreter. For example, builtins and sys modules no longer copy the module dictionary of the main interpreter, but create their own dictionary from scratch.

See each commit for the details.

Py_Finalizer() could share more code with Py_EndInterpreter(), but each Py_Finalizer() change is really tricky and require to pay a lot attention. The Python finalization is really fragile. I started to take notes on this fragile code:
https://pythondev.readthedocs.io/finalization.html

I consider that the initial issue is fixed, so I close the issue.