CLN: Specialize assert_(np.array_equal(...))

Work on #6175. Work in progress. One more commit needed to finish; splitting to keep reasonably sized.

* Changes assert_numpy_array_equals to assert_numpy_array_equal, for consistency with other methods. (API change of recently-added function)
* Changes instances of assert_(np.array_equal(A,B)) to assert_numpy_array_equal(A,B)
* Update release.rst to reflect work on #6175
Merge pull request #6368 from bwignall/assert_6175_arrayequal_1

CLN: Specialize assert_(np.array_equal(...))