Remove some unnecessary uses of scipy

[alexfikl]

1. Replaced the uses of scipy.special.factorial with mi_factorial in coefficient tests. Hopefully fixes https://gitlab.tiker.net/inducer/sumpy/-/jobs/957466.
2. Replaced uses of scipy.fftpack with numpy.fft.
3. Replaced some uses of Legendre and Chebyshev functions in point_calculus.

[Copilot]

Pull request overview

This PR reduces SciPy usage by replacing selected SciPy helpers with NumPy or existing sumpy utilities in tests and point calculus code.

Changes:

• Replaced scipy.special.factorial usage in coefficient tests with multi-index helpers.
• Replaced scipy.fftpack.diff with NumPy FFT-based differentiation.
• Replaced SciPy Legendre/Chebyshev helpers and corrected a documentation typo.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sumpy/test/test_m2m_coeffs.py	Uses existing multi-index helpers for powers, sums, and factorials.
sumpy/test/test_l2l_coeffs.py	Uses existing multi-index helpers for powers, sums, and factorials.
sumpy/test/geometries.py	Replaces SciPy FFT differentiation in torus geometry generation.
sumpy/test/curve.py	Adds NumPy FFT-based periodic differentiation for curve geometry.
sumpy/point_calculus.py	Replaces SciPy Legendre and Chebyshev routines with NumPy-based code.
doc/misc.rst	Corrects the project name in acknowledgments.
.basedpyright/baseline.json	Updates static type-checking baseline entries.

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[inducer]

Thanks!