mkl_fft requires scipy when import `mkl_fft.interfaces.scipy_fft`

[ekomarova]

mkl_fft requires scipy when import mkl_fft.interfaces.scipy_fft in the env w/o scipy:

>> conda creare -n test_fft mkl_fft numpy -c gold
>> python
>>> import mkl_fft.interfaces.scipy_fft
Traceback (most recent call last):
    from ._scipy_fft import (
    import scipy
ModuleNotFoundError: No module named 'scipy'

Module scipy_fft does not have the simple check for the presence of scipy in the environment, which can lead to such a problem. Initially, it was expected that fft should work without scipy

[ndgrigorian]

@ekomarova
This was an intentional change.

Since the interface is only really needed as drop-in scipy replacement, we are only testing it with scipy in the environment. Additionally, it uses functionality from scipy (we import fftfreq, rfftfreq, fftshift, and ifftshift from scipy) there would be issues importing without scipy anyway.

[ekomarova]

The first stop of the problem is recipe https://github.com/IntelPython/mkl_fft/blob/master/conda-recipe/meta.yaml#L42, not only other tests

So technically it's not a problem if user doesn't import such a module without scipy, but in any case we either need to re-organize the interface or as min the recipe