Revert "i18n doctest was missing an import"

This reverts commit 45c4c4eb93ea6f60450d3dfe7cc0dcec9b08dcbe.

I do not see the sense of the previous commit.
fyflake does sometimes report nonsense. To test the method you
have to import the module(s) (which is trivial) but in the right
form:

>>> import pwb  # if pywikibot is not a side package
>>> import pywikibot
>>> from pywikibot import i18n

(and if really necessary, place it obove the comment line)

Change-Id: Ie3c765f1520c6585f8e22f72c942ca7da0ebae75