Issue 17862:  Improve the signature of itertools grouper() recipe.
Issue 17862:  Improve the signature of itertools grouper() recipe.

Putting *n* after the *iterable* matches the signature of other itertools
and recipes.  Also, it reads better.

Suggested by Ezio Melotti.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator