ideal.py doctest failure

<div id="comment:0"></div>


```
File "ideal.py", line 384:
    sage: I.is_prime()
Expected:
    Traceback (most recent call last):
    ...
    NotImplementedError
Got:
    True
```


Component: **group theory**

_Issue created by migration from https://trac.sagemath.org/ticket/2457_

