* DOC: Note FFT type promotion

The NumPy FFT implementation will promote `float32` types to `float64` types. This may not be desired for some applications, but the SciPy implementation supports more data types.

Closes #14892

These aren't public, and should have been added with underscores.

pytesttester is used by every single subpackage, so making it depend on np.testing just creates cyclic dependencies that can lead to circular imports

Relates to #11457

Numpy can now be tested using the standard

    `python -c"import numpy; numpy.test()"`

construct.

The "bench" testing with the old bench files is no longer supported.
These days we use `runtests.py` and `asv`.

The aim here is to separate out the nose dependent files prior to adding
pytest support. This could be done by adding new files to the general
numpy/testing directory, but I felt that it was to have the relevant
files separated out as it makes it easier to completely remove nose
dependencies when needed.

Many places were accessing submodules in numpy/testing directly, and in
some cases incorrectly. That presented a backwards compatibility
problem. The solution adapted here is to have "dummy" files whose
contents will depend on whether of not pytest is active. That way the
module looks the same as before from the outside.

In the case of numpy itself, direct accesses have been fixed. Having
proper `__all__` lists in the submodules helped in that.

Add `print_function` to all `from __future__ import ...` statements
and use the python3 print function syntax everywhere.

Closes #3078.