BUG: ndimage: return signed dtype from label by default

Change default return dtype from ndimage.label back to signed integer,
as find_objects and some other functions don't support unsigned intp.
DOC: add 0.13.1 release notes.
REL: update release scripts for 0.13.1, fix some minor OS X issues.

Numpy 1.5.1 doesn't work for Python 3.x with scipy, use 1.7.1 instead.
Note that this was also done for the 0.13.0 release.
REL: set version to 0.13.1