bpo-43770: Refactor PyType_Ready() function (GH-25336)

* Split PyType_Ready() into sub-functions.
* type_ready_mro() now checks if bases are static types earlier.
* Check tp_name earlier, in type_ready_checks().
* Add _PyType_IsReady() macro to check if a type is ready.

Include/internal/pycore_object.h

@@ -174,6 +174,10 @@
     const char *slot_name,
     int success);
 
+// PyType_Ready() must be called if _PyType_IsReady() is false.
+// See also the Py_TPFLAGS_READY flag.
+#define _PyType_IsReady(type) ((type)->tp_dict != NULL)
+
 #ifdef __cplusplus
 }
 #endif