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/Python/_warnings.c b/Python/_warnings.c
index d865f0a..36d649f 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -1185,10 +1185,9 @@
 }
 
 static PyObject *
-init_filters(void)
+init_filters(const _PyCoreConfig *config)
 {
-    PyInterpreterState *interp = PyThreadState_GET()->interp;
-    int dev_mode = interp->core_config.dev_mode;
+    int dev_mode = config->dev_mode;
 
     Py_ssize_t count = 2;
     if (dev_mode) {
@@ -1264,8 +1263,8 @@
 };
 
 
-PyMODINIT_FUNC
-_PyWarnings_Init(void)
+PyObject*
+_PyWarnings_InitWithConfig(const _PyCoreConfig *config)
 {
     PyObject *m;
 
@@ -1274,7 +1273,7 @@
         return NULL;
 
     if (_PyRuntime.warnings.filters == NULL) {
-        _PyRuntime.warnings.filters = init_filters();
+        _PyRuntime.warnings.filters = init_filters(config);
         if (_PyRuntime.warnings.filters == NULL)
             return NULL;
     }
@@ -1305,3 +1304,12 @@
     _PyRuntime.warnings.filters_version = 0;
     return m;
 }
+
+
+PyMODINIT_FUNC
+_PyWarnings_Init(void)
+{
+    PyInterpreterState *interp = PyThreadState_GET()->interp;
+    const _PyCoreConfig *config = &interp->core_config;
+    return _PyWarnings_InitWithConfig(config);
+}