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