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.

Programs/_testembed.c

@@ -603,6 +603,8 @@ static int test_init_from_config(void)
     Py_FrozenFlag = 0;
     config.pathconfig_warnings = 0;
 
+    config._isolated_interpreter = 1;
+
     init_from_config_clear(&config);
 
     dump_config();