mp_obj_print_helper: Handle a ctrl-c that comes in during printing

In adafruit#2689, hitting ctrl-c during the printing of an object with a lot of sub-objects could cause the screen to stop updating (without showing a KeyboardInterrupt).  This makes the printing of such objects acutally interruptable, and also correctly handles the KeyboardInterrupt:

```
>>> l = ["a" * 100] * 200
>>> l
['aaaaaaaaaaaaaaaaaaaaaa...aaaaaaaaaaa', Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyboardInterrupt:
>>>
```

Author: jepler

py/obj.c

@@ -67,6 +67,8 @@ void mp_obj_print_helper(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
     #ifdef RUN_BACKGROUND_TASKS
     RUN_BACKGROUND_TASKS;
     #endif
+    mp_handle_pending();
+
 #ifndef NDEBUG
     if (o_in == MP_OBJ_NULL) {
         mp_print_str(print, "(nil)");