Python: clear Py-driver variables on exit

These kinds of leaks are relatively harmless, it reduces the number of
un-freed data reported by valgrind on exit.

source/blender/python/BPY_extern.h

@@ -69,6 +69,11 @@ void BPY_modules_load_user(struct bContext *C);
 
 void BPY_app_handlers_reset(bool do_all);
 
+/**
+ * Run on exit to free any cached data.
+ */
+void BPY_driver_exit(void);
+
 /**
  * Update function, it gets rid of python-drivers global dictionary: `bpy.app.driver_namespace`,
  * forcing #BPY_driver_exec to recreate it. Use this when loading a new `.blend` file