Fixes gh-15325

* BUG, TST: fix f2py for PyPy, skip one test for PyPy, xfail tests for s390x

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

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

Use standard pytest markers everywhere in the numpy tests. At this point
there should be no nose dependency. However, nose is required to test
the legacy decorators if so desired.

At this point, numpy test cannot be run in the way with runtests, rather
installed numpy can be tested with `pytest --pyargs numpy` as long as
that is not run from the repo. Run it from the tools directory or some
such.

1. fail tests related to DLL load failure as they were previously untested.
2. fix have_compiler to return false on old compilers
3. xfail some tests that were not working on old Python versions.

This allows pytest to run with duplicate test file names. Note that
`python <path-to-test-file>` no longer works with this change, nor will
a simple `pytest numpy`, because numpy is imported from the numpy
repository. However, `python runtests.py` and `>>> numpy.test()` are
still available.