Co-authored-by: Melissa Weber Mendonça <melissawm@gmail.com>

Co-authored-by: Charles Harris <charlesr.harris@gmail.com>

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.

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.

This augments the test in constant_compound.f90 by
using constants without a compound kind spec to
illustrate the case that led to the
reporting of issue #8493

This is a test code provided as a patch by @zerothi checking
the compound constant parsing.