Return palette contents in suitable format for C putpalette()

[radarhere]

ImagePalette.getdata() does not always fulfil its described purpose.

Pillow/src/PIL/ImagePalette.py

Lines 100 to 103 in 0253ef0

	def getdata(self) -> tuple[str, Sequence[int] | bytes | bytearray]:
	"""
	Get palette contents in format suitable for the low-level
	``im.putpalette`` primitive.

from PIL import Image, ImagePalette
im = Image.new("P", (1, 1))
palette = ImagePalette.raw("RGB", list(range(256)) * 3)
rawmode, data_out = palette.getdata()
im.im.putpalette("RGB", rawmode, data_out)

currently raises

Traceback (most recent call last):
  File "demo.py", line 5, in <module>
    im.im.putpalette("RGB", rawmode, data_out)
TypeError: a bytes-like object is required, not 'list'

This PR ensures that the palette data is always returned as bytes.