bpo-32030: Add pymain_get_global_config() (#4735)

* Py_Main() now starts by reading Py_xxx configuration variables to
  only work on its own private structure, and then later writes back
  the configuration into these variables.
* Replace Py_GETENV() with pymain_get_env_var() which ignores empty
  variables.
* Add _PyCoreConfig.dump_refs
* Add _PyCoreConfig.malloc_stats
* _PyObject_DebugMallocStats() is now responsible to check if debug
  hooks are installed. The function returns 1 if stats were written,
  or 0 if the hooks are disabled. Mark _PyMem_PymallocEnabled() as
  static.

Python/sysmodule.c

@@ -1368,8 +1368,7 @@ static PyObject *
 sys_debugmallocstats(PyObject *self, PyObject *args)
 {
 #ifdef WITH_PYMALLOC
-    if (_PyMem_PymallocEnabled()) {
-        _PyObject_DebugMallocStats(stderr);
+    if (_PyObject_DebugMallocStats(stderr)) {
         fputc('\n', stderr);
     }
 #endif