bpo-42260: PyConfig_Read() only parses argv once (GH-23168)

The PyConfig_Read() function now only parses PyConfig.argv arguments
once: PyConfig.parse_argv is set to 2 after arguments are parsed.
Since Python arguments are strippped from PyConfig.argv, parsing
arguments twice would parse the application options as Python
options.

* Rework the PyConfig documentation.
* Fix _testinternalcapi.set_config() error handling.
* SetConfigTests no longer needs parse_argv=0 when restoring the old
  configuration.

Modules/_testinternalcapi.c

@@ -253,14 +253,17 @@ test_set_config(PyObject *Py_UNUSED(self), PyObject *dict)
     PyConfig config;
     PyConfig_InitIsolatedConfig(&config);
     if (_PyConfig_FromDict(&config, dict) < 0) {
-        PyConfig_Clear(&config);
-        return NULL;
+        goto error;
     }
     if (_PyInterpreterState_SetConfig(&config) < 0) {
-        return NULL;
+        goto error;
     }
     PyConfig_Clear(&config);
     Py_RETURN_NONE;
+
+error:
+    PyConfig_Clear(&config);
+    return NULL;
 }