Some more Python 2 cleanup

The kwarg option norm=inverse leads to scaling of the transforms
inverse (opposite) to that of the default option norm=None; i.e. the
forward transform is normalized with 1/n whereas the backward one
with 1. The fft routines and their tests have been modified to
reflect the changes; all tests have been passed successfully.

Closes #16126

* modified _unitary, fft & ifft and their tests; tests passed

* modified rfft & irfft and their tests; tests passed

* modified hfft & ihfft and all tests; all modifications are now done

* correction in ihfft; all 79 fft tests passed successfully

* undo unnecessary docstrings changes made in my previous commits

* use norm=forward kwarg value name

* Update numpy/fft/__init__.py

Co-authored-by: Leo Fang <leofang@bnl.gov>

* add code review suggestions

Co-authored-by: Leo Fang <leofang@bnl.gov>

* add default norm=None alias norm=backward

* streamline private normalization functions

* modify hermitian FFTs

* add review suggestions

Co-authored-by: Leo Fang <leofang@bnl.gov>

* add review suggestions v2 (dict as module const)

* make review suggestions v3

* Apply suggestions from code review

Co-authored-by: Leo Fang <leofang@bnl.gov>

* 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)

Inheriting from object was necessary for Python 2 compatibility to use
new-style classes. In Python 3, this is unnecessary as there are no
old-style classes.

Dropping the object is more idiomatic Python.

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