made HUSL diff test fixture more flexible
made HUSL diff test fixture more flexible

since this project no longer aims to produce completely accurate RGB->HUSL->RGB transformations,
the _diff_husl test fixture shouldn't check for exact RGB values. The Cython, NumPy, and NumExpr
impls will be exact, but the C impl may be off by as much as one three percent for high saturation
values. The HUSL-RGB conversions are still accurate enough that you can't pick out differences
by eye.

TadLeonard/husl-numpy,TadLeonard/husl-numpy,TadLeonard/husl-numpy