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

* Review comments

* More tests in test_installed_npymath_ini

* Review comments

After the pytest migration, test classes no longer inherit
from unittest.TestCase and and the fail method does not
exist anymore.

In all these cases, we can use assert_raises and assert_raises_regex instead

Pytest < 3.8 ignored warnings issued during test collection, but that
changed in pytest 3.8 and the method NumPy used to suppress the
PendingDeprecationWarning for matrices no longer worked, or rather, was
exposed as not working. The fix here is to suppress the warning in
pytest.ini and pytesttester.py , which should work as long as the tests
are the only places left where NumPy uses matrices.

An alternate fix is to delay the construction of matrices in the tests
until they are actually run, which has the virtue of test localization
but is a bit more complicated.

See https://github.com/pytest-dev/pytest/issues/3945 for discussion.

The docstring already assumed it was in linalg, and this ensures
linalg becomes completely independent of matrixlib.

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

Found via `codespell -q 3 -I ../numpy-whitelist.txt`

Abstract collection classes accessed from the collections module
have been deprecated since Python 3.3. They should be
accessed through collections.abc. When run with Python
3.7, the deprecation warning cause multiple tests to
fail.

Fixes gh-9506, unsigned exponentiation