* Review F401,F841,F842 flake8 errors (unused variables, imports)

* Review comments

* More tests in test_installed_npymath_ini

* Review comments

Further progress in isolating matrix in preparation of its
deprecation.  There is one place left with an explicit reference
to matrix (in MaskedArray.count), which is to be solved later.

That function is nose specific and has not worked since `__init__` files
were added to the tests directories.

Depending on the input, this would return:
* A single slice, if mask=nomask
* A list of slices, if mask is an array
* None, if mask is fully masked

The documented return value is a list, and all downstream callers of this
function end up having to correct for it not being one.

This affects the result of np.ma.notmasked_contiguous, which also did not
document these unusual return values.

[ci skip]

Uses '_fromnxfunction_seq' just like 'vstack', 'hstack', etc.
Added release note to 1.15.0.