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.
BUG: ndimage: use only normalized type_num in comparisons

Numpy type_num cannot be used in comparisons, since some of the values
are equivalent, depending on the platform.
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
REL: set version to 0.13.2-dev
TST: check number of returned regions

added check on `n` for all tests of label()

(cherry picked from commit bcb81c005eaaa649c596ccfa86f87f91910219c9)
BUG: fix number of regions in case where no regions are found

Added special-case logic for the case were no regions were found so
that 0 is returned instead of 1

Fixes issue #3082

(cherry picked from commit fba0496ebd75b7656864184eb8e73d6b466e855b)