lint: Test and improve PNG transparency check (f-019)

[gvtulder]

se lint doesn't recognize some types of PNG transparency. This adds small test images with different types of transparency/non-transparency, and some additional code to cover:

• PNG image.mode == "LA": grayscale with an alpha channel. Similar to the RGBA that was already supported.
• PNG image.mode == "P": a palette-based images, but with multiple transparency levels. This means that image.info["transparency"] is a bytes string with an 0-255 alpha level for every indexed color. The current version of has_transparency only expected one single transparent index.

The test includes some transparent and some non-transparent images, where some of the non-transparent images have an unused alpha channel.

PIL.Image details per image:

	mode	info[transparency]	Transparent?
illustration-1.png	L	None	No
illustration-2.png	LA	None	Yes
illustration-3.png	P	b'\x00\x11"3DUfw\x88\x99\xaa\xbb\xcc\xdd\xee'	Yes
illustration-4.png	P	0	Yes
illustration-5.png	RGBA	None	Yes
illustration-6.png	RGBA	None	No
illustration-7.png	RGB	None	No
illustration-8.png	LA	None	No

[acabal]

Great, thanks!