DXT3 images are read in RGBA mode

[radarhere]

https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#dds

Added in version 3.4.0: DXT3 images can be read in RGB mode

This is incorrect. DXT3 images are read in RGBA mode

Pillow/src/PIL/DdsImagePlugin.py

Lines 390 to 391 in 5efcaa4

	elif fourcc == D3DFMT.DXT3:
	self._mode = "RGBA"

Pillow/Tests/test_file_dds.py

Lines 68 to 72 in 5efcaa4

	with Image.open(TEST_FILE_DXT3) as im:
	im.load()
	assert im.format == "DDS"
	assert im.mode == "RGBA"

This has been the case since #2068