bpo-36142: _PyPreConfig_Write() sets the allocator (GH-12186)

* _PyPreConfig_Write() now sets the memory allocator.
* _PyPreConfig_Write() gets a return type: _PyInitError.
* _Py_InitializeCore() now reads and writes the pre-configuration
  (set the memory allocator, configure the locale) before reading and
  writing the core configuration.

Modules/main.c

@@ -304,8 +304,7 @@ preconfig_read_write(_PyPreConfig *config, const _PyArgv *args)
         return err;
     }
 
-    _PyPreConfig_Write(config);
-    return _Py_INIT_OK();
+    return _PyPreConfig_Write(config);
 }