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

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

* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

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

Create new file `test_compile_function.py` for the test, transferring
the content from `test_quoted_characters.py` where it was before. The
tricky part here is maintaining the history in both files after the
move.

* replaced the usage of exec_command() in the
f2py init with standard library equivalent

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.