bpo-34170: Add _PyCoreConfig.isolated (GH-8417)

* _PyCoreConfig: add isolated and site_import attributes
* Replace Py_IgnoreEnvironment with config->ignore_environment when
  reading the current configuration
* _PyCoreConfig_Read() now sets ignore_environment, utf8_mode,
  isolated and site_import from Py_IgnoreEnvironment, Py_UTF8Mode,
  Py_IsolatedFlag and Py_NoSiteFlag
* _Py_InitializeCore() now sets Py_xxx flags from the configuration
* pymain_read_conf() now uses _PyCoreConfig_Copy() to save/restore
  the configuration.
* Rename _disable_importlib of _PyCoreConfig to _install_importlib
* _PyCoreConfig_SetGlobalConfig() now also set
  Py_HashRandomizationFlag
* Replace !Py_NoSiteFlag with core_config->site_import
diff --git a/Python/pathconfig.c b/Python/pathconfig.c
index 1f6177f..509ea8e 100644
--- a/Python/pathconfig.c
+++ b/Python/pathconfig.c
@@ -283,8 +283,7 @@
 
 
 _PyInitError
-_PyCoreConfig_InitPathConfig(_PyCoreConfig *config,
-                             int *isolated, int *no_site_import)
+_PyCoreConfig_InitPathConfig(_PyCoreConfig *config)
 {
     _PyPathConfig path_config = _PyPathConfig_INIT;
     _PyInitError err;
@@ -345,11 +344,11 @@
         }
     }
 
-    if (path_config.isolated != -1 && isolated != NULL) {
-        *isolated = path_config.isolated;
+    if (path_config.isolated != -1) {
+        config->isolated = path_config.isolated;
     }
-    if (path_config.no_site_import != -1 && no_site_import != NULL) {
-        *no_site_import = path_config.no_site_import;
+    if (path_config.site_import != -1) {
+        config->site_import = path_config.site_import;
     }
 
     _PyPathConfig_Clear(&path_config);
@@ -375,10 +374,7 @@
     _PyInitError err;
     _PyCoreConfig config = _PyCoreConfig_INIT;
 
-    /* Py_IsolatedFlag and Py_NoSiteFlag are left unchanged: pass NULL.
-       _PyCoreConfig_InitPathConfig() will be called later and will set
-       these flags. */
-    err = _PyCoreConfig_Read(&config, NULL, NULL);
+    err = _PyCoreConfig_Read(&config);
     if (_Py_INIT_FAILED(err)) {
         goto error;
     }