Stable API for scipy FFT backend

[bombaci-vsc]

#45 introduced _scipy_fft_backend.py which could be used as an argument to scipy.fft.set_backend().

import sicpy.fft, mkl_fft._scipy_fft_backend as mkl_be
scipy.fft.set_backend(mkl_be)

In v1.3.12 _scipy_fft_backend.py was renamed to _scipy_fft.py. While this makes the interface finally match the example in the changelog, it breaks any code that was previously importing _scipy_fft_backend.

Being able to change the scipy fft backend is a very useful feature. Would it be possible to formally support it by having a stable API for accessing the module and documenting the backend support in the README.md?