bpo-32030: Add _PyMainInterpreterConfig.pythonhome (#4513)

* Py_Main() now reads the PYTHONHOME environment variable
* Add _Py_GetPythonHomeWithConfig() private function
* Add _PyWarnings_InitWithConfig()
* init_filters() doesn't get the current core configuration from the
  current interpreter or Python thread anymore. Pass explicitly the
  configuration to _PyWarnings_InitWithConfig().
* _Py_InitializeCore() now fails on _PyWarnings_InitWithConfig()
  failure.
* Pass configuration as constant
diff --git a/Include/pystate.h b/Include/pystate.h
index b2739f1..ab6400c 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -61,12 +61,15 @@
 typedef struct {
     int install_signal_handlers;
     wchar_t *module_search_path_env; /* PYTHONPATH environment variable */
+    wchar_t *pythonhome;    /* PYTHONHOME environment variable,
+                               see also Py_SetPythonHome(). */
 } _PyMainInterpreterConfig;
 
 #define _PyMainInterpreterConfig_INIT \
     (_PyMainInterpreterConfig){\
      .install_signal_handlers = -1, \
-     .module_search_path_env = NULL}
+     .module_search_path_env = NULL, \
+     .pythonhome = NULL}
 
 typedef struct _is {