Fixed combining single duration across duplicate APNG frames

[radarhere]

Resolves #7144

When saving APNG files, we are testing that when duplicate frames are combined, the different durations are added together.

Pillow/Tests/test_file_apng.py

Lines 433 to 441 in 2a274a4

	# test removal of duplicated frames
	frame = Image.new("RGBA", (128, 64), (255, 0, 0, 255))
	frame.save(
	test_file, save_all=True, append_images=[frame, frame], duration=[500, 100, 150]
	)
	with Image.open(test_file) as im:
	im.load()
	assert im.n_frames == 1
	assert im.info.get("duration") == 750

However, we aren't testing that the durations are combined when the duration keyword is a single number, rather than a list or a tuple. This issue found that this situation fails, and this PR corrects that.

[gsingh93]

I've been using a local build with this patch for a few weeks now and it's been working well, hopefully we can get this merged soon :)

[radarhere]

Be aware that even if it is merged, Pillow 10.0.0 is not due out until July 1.

[gsingh93]

Yup, just looking forward to being able to remove the applying of these patches from my CI infrastructure and providing patching instructions to anyone who wants to run my code. I'm fine using master for now, other than that.

[radarhere]

Pillow 10.0.0 has now been released with this fix.