Do yourself a favor and don't become a PEP 8 nut ;-)

The primary purpose of the example in whatsnew is to teach, demonstrate, and explain.  Any PEP 8 considerations take a back seat to that purpose (even PEP 8 itself admonishes the reader with the famous foolish consistency quotation).

The purpose behind the usual space conventions for keyword arguments is to allow them to be visually grouped when more than one is used on a line.  In the case of the argparse example, the important parameters are being listed one per line along with an explanatory comment.  In my judgement, the example is more readable with the spacing as it allows the eye to easily pick-out the argument list.  

Am sorry, but it is not really open for debate.  Whatsnew is a single-author document reflecting an enormous amount of my time culling changes, crafting expository text, and creating strong examples.  As such, it is going to very much reflect my style and judgement.