DOC/CI: add missing import to ipython directive in whatsnew/v0.11.0.rst (#40937)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: attack68

doc/source/whatsnew/v0.11.0.rst

@@ -306,6 +306,7 @@ Astype conversion on ``datetime64[ns]`` to ``object``, implicitly converts ``NaT
 
 .. ipython:: python
 
+   import datetime
    s = pd.Series([datetime.datetime(2001, 1, 2, 0, 0) for i in range(3)])
    s.dtype
    s[1] = np.nan