Improve consistency of colorsys.rgb_to_hsv

[seb-hub]

Cache repeated difference to make code easier to read and consistent with colorsys.rgb_to_hls.

[the-knights-who-say-ni]

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[nanjekyejoannah]

LGTM, thanks @seb-hub

[seb-hub]

Thanks for the fast review, @nanjekyejoannah and @Fidget-Spinner.

[Fidget-Spinner]

@seb-hub congrats on landing your first PR in cpython. Hope to see you around :).

[seb-hub]

@Fidget-Spinner: It's been quite exciting for me :-)
I will look out for other possibilities to contribute!