Fix unrecognized 'Z' UTC designator

closes #8771 
Totally agree and as I understand it this is exactly the behavior of the parser in np_datetime_strings.c *but* there is currently a fallback (dateutil's parser) if anything goes wrong and this is what we are actually testing with `...Z0` and `...Z00`. I think this fallback is there for a reason so I did not want to mess with it