a pend item could be removed (https://github.com/python/cpython/blob/master/Python/_warnings.c#L493).

two reasons:
1) every warning have `__name__` and it must not NULL(`The tp_name slot must be set;` from pep0253)
2) the `__name__` of Warning class(including children class) can not be removed.

```
>>> del UserWarning.__name__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'UserWarning'
```