Do not open YCbCr colorspace images as RGB without libtiff

[radarhere]

TIFF images in the YCbCr colorspace (6) are currently opened as RGB.

Pillow/src/PIL/TiffImagePlugin.py

Lines 270 to 273 in 1219c27

	# JPEG compressed images handled by LibTiff and auto-converted to RGBX
	# Minimal Baseline TIFF requires YCbCr images to have 3 SamplesPerPixel
	(II, 6, (1,), 1, (8, 8, 8), ()): ("RGB", "RGBX"),
	(MM, 6, (1,), 1, (8, 8, 8), ()): ("RGB", "RGBX"),

However, TiffImagePlugin does not always use libtiff. This PR only applies the RGB treatment when libtiff is used.