bpo-42639: atexit now logs callbacks exceptions (GH-23771)

At Python exit, if a callback registered with atexit.register()
fails, its exception is now logged. Previously, only some exceptions
were logged, and the last exception was always silently ignored.

Add _PyAtExit_Call() function and remove
PyInterpreterState.atexit_func member. call_py_exitfuncs() now calls
directly _PyAtExit_Call().

The atexit module must now always be built as a built-in module.

Include/internal/pycore_interp.h

@@ -233,8 +233,8 @@ struct _is {
     PyObject *after_forkers_parent;
     PyObject *after_forkers_child;
 #endif
+
     /* AtExit module */
-    void (*atexit_func)(PyObject *);
     PyObject *atexit_module;
 
     uint64_t tstate_next_unique_id;

Include/internal/pycore_pylifecycle.h

@@ -109,6 +109,8 @@ PyAPI_FUNC(void) _PyErr_Display(PyObject *file, PyObject *exception,
 
 PyAPI_FUNC(void) _PyThreadState_DeleteCurrent(PyThreadState *tstate);
 
+extern void _PyAtExit_Call(PyObject *module);
+
 #ifdef __cplusplus
 }
 #endif