Update scipy requirement to 1.4.1 and use scipy.fft instead of scipy.fftpack

[srvanrell]

Description

• Update requirement of scipy as suggested in isssue EXAMPLES: Change use of scipy.fftpack to scipy.fft #5986.
• Update examples importing fftpack to use fft instead
• Remove _shared.fft workaround that loaded numpy.fft, scipy.fft or scipy.fftpack
• Update imports were necessary to use scipy.fft module

Closes #5986

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @srvanrell! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/examples/registration/plot_register_rotation.py:

Line 135:1: E402 module level import not at top of file

Comment last updated at 2021-11-09 00:51:55 UTC

[grlee77]

Thanks. The test failures here seem to be unrelated (They appear to be an issue with a recent release of setuptools)

[srvanrell]

@grlee77 The PR looks complete for me but you mentioned in #5986 that there were some calls to astype that are no longer needed. Let me know if there is something more to do or fix. Thank you for your patience with a newbie.

[grlee77]

@grlee77 The PR looks complete for me but you mentioned in #5986 that there were some calls to astype that are no longer needed. Let me know if there is something more to do or fix. Thank you for your patience with a newbie.

Yes, I will take a look at that now and push any changes here.

[grlee77]

Thanks @srvanrell, this looks good!

[hmaarrfk]

Thanks you two!