Improved HSV conversion

[radarhere]

Resolves #3997

The issue reports that you can't convert from P to HSV, whereas this previously worked.

If a direct conversion from a mode to another is not possible, then Pillow attempts to convert the image to its base mode first, and then convert from that intermediate image.

Pillow/src/PIL/Image.py

Lines 1050 to 1056 in f3f45cf

	try:
	im = self.im.convert(mode, dither)
	except ValueError:
	try:
	# normalize source image and try again
	im = self.im.convert(getmodebase(self.mode))
	im = im.convert(mode, dither)

PR #3726 changed the base mode of P from RGB to P. So while Pillow previously worked around the absence of a direct method for converting from P to HSV, it now throws an error.

This PR fixes that by assigning direct methods for conversion from P to HSV, as well as 1 to HSV, L to HSV, LA to HSV, I to HSV, RGBA to HSV, RGBX to HSV and CMYK to HSV.

[hugovk]

Let's bracket these single-liners for safety.

[radarhere]

Okay, done