When converting RGBA to PA, use RGB to P quantization

[radarhere]

Resolves #9151 with two changes.

1. Currently,

p = hopper("P")
a = Image.new("L", p.size)
pa = Image.merge("PA", (p, a))
print(pa.palette.colors)

returns populated palette data, but

print(pa.getpalette())

is empty. This fixes that.

2. When converting RGBA to PA, rather than using the C method topalette(), quality can be improved by using the Python quantize() to convert the RGB channels to P, and copying the A channel exactly by using split() and merge().

[bryghtlabs-richard]

LGTM.