bpo-40453: Add PyConfig._isolated_subinterpreter (GH-19820)

An isolated subinterpreter cannot spawn threads, spawn a child
process or call os.fork().

* Add private _Py_NewInterpreter(isolated_subinterpreter) function.
* Add isolated=True keyword-only parameter to
  _xxsubinterpreters.create().
* Allow again os.fork() in "non-isolated" subinterpreters.

Modules/_posixsubprocess.c

@@ -663,6 +663,14 @@ subprocess_fork_exec(PyObject* self, PyObject *args)
         return NULL;
     }
 
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+    if (config->_isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "subprocess not supported for isolated subinterpreters");
+        return NULL;
+    }
+
     /* We need to call gc.disable() when we'll be calling preexec_fn */
     if (preexec_fn != Py_None) {
         PyObject *result;