As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

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

* Review comments

* More tests in test_installed_npymath_ini

* Review comments

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.

This is in preparation for deprecation and eventual removal
of the matrix class.