BUG: Update 1.17.x with 1.18.0-dev pocketfft.py.

[charris]

Backport of #14279,

BUG: fix issue #14176

This replaces the fct variable in pocketfft.py by its inverse, inv_fct. The actual factor is only built immediately before executing the FFT. This defers potential divide-by-zero exceptions until after the check for zero-length transforms.

The easiest way to backport #14279 was to copy pocketfft.py from master.
That also brings some improved documentation and other fixes with it, which
cannot hurt.

[rgommers]

Noticing this only now and not specific to this PR but: yet again missing underscores in a filename. dir(np.fft) shows pocketfft and pocketfft_internal, that should be fixed up and preferably also backported to 1.17.x

[charris]

@rgommers Is it your suggestion that pocketfft_internal should be renamed _pocketfft?

[rgommers]

No, pocketfft.py should be renamed _pocketfft.py, and pocketfft_internal should be _pocketfft_internal. The rule is simple: those things are private, so must start with an underscore.

[charris]

To be consistent, then we should probably rename pocketfft.c to pocketfft_internal.c

[grlee77]

Thanks for backporting this @charris!