REL: Prepare for 1.16.6 release.

- Update 1.16.6-notes.rst
- Create 1.16.6-changelog.rst

ENH: Backport improvements to testing functions.

This improves the documentation and error messages of the testing
functions and extends `numpy.testing.assert_array_compare` to
more data types.

BUG: lib: Fix histogram problem with signed integer arrays.

BLD: Prevent -flto from optimising long double representation check away

An input such as

    np.histogram(np.array([-2, 0, 127], dtype=np.int8), bins="auto")

would raise the exception

    ValueError: Number of samples, -1, must be non-negative.

The problem was that the peak-to-peak value for the input array was
computed with the `ptp` method, which returned negative values for
signed integer arrays when the actual value was more than the
maximum signed value of the array's data type.

The fix is to use a peak-to-peak function that returns an
unsigned value for signed integer arrays.

Closes gh-14379.