Fix unrecognized 'Z' UTC designator

closes #8771 
not sure where you got the idea that I was suggesting you modify ``convert_to_tsobject``. That's very complicated and not warranted. I suggest that you make a simple modification in the *same* file you are currently working, ``np_datetime_strings.c`` to handle the case of reading 1 and 2 characters past the Z.  

I would maybe make ``out_local==-1`` an error, then in the except block, if out_local is -1 you can simply re-raise the ValueError (in ``convert_to_tsobject``)

or you can put a specific message in the ``ValueError`` which is checked in ``convert_to_tsobject``. You are right, the problem is a ``ValueError`` actually means 2 things here. Need to disambiguate them (or allow your change, and simply disregard the 'error' cases that we had before).