graphics/py-spectra: Color scales and color conversion

Spectra is a Python library that makes color math, color scales, and
color-space conversion easy.
Support for:
    Color scales
    Color ranges
    Color blending
    Brightening/darkening colors
    Saturating/desaturating colors
    Conversion to/from multiple color spaces
Spectra is built on colormath and grapefruit. Spectra is enormously inspired by
chroma.js and d3's scales.

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