Set pixels using slice with single color / multiple colors

Playing around with NeoPixel I thought of building some stuff on top using the slice syntax
i.e `pixels[:len(pixels)//2] = (255, 0 ,0)`
Which currently fails as the colors tuple must be at least the length of the slice

Using `pixels.fill((255,0,0)` could work, but only if you start at the beginning, not for arbitrary slices (i.e setting middle third with `third = len(pixels)//3` then `pixels[third: 2 * third] = (255, 0, 0)`

Bonus: if a tuple is given, but not of the same length, It could be cool if it just repeated, meaning:
`pixels[:7] = (R, G, B)` would set the pixels `(R, G, B, R, G, B, R)`

Since this would have thrown an error before, it should be backwards-compatible.

I realize all this can be done manually but since slices are already supported it seems like a nice intuitive feature :)

Start by locating the existing NeoPixel slice-assignment implementation and the tests covering slice writes. Define the behavior for assigning one RGB tuple to an arbitrary slice, including the proposed repeated-tuple case, and consider compatibility with previously invalid assignments. Done means the requested slice examples work without breaking existing slice behavior.

Written by the indexing model from the issue text.