Drop pyfftw from requirements
After replacing the numpy.fft implementation with the faster one from scipy (!55 (merged), #99 (closed)), I just ran a test with 10000 tie points using the pyfftw implementation. It turned out that the newly implementated scipy version is even slightly faster than pyfftw.
Since pyfftw>=0.13 is currently unusable for AROSICS due to this issue and pyfftw<=0.12 does not support Python>=3.11, I see no reason to still keep pyfftw in the requirements. The above pyfftw issue is already open since 4 years with no change and also the pending Python 3.12 rebuild for the latest pyfftw version (this PR) blocks AROSICS from being rebuilt for Python 3.12 (#96 (closed)).