ValueError: cannot add non-opaque RGBA color to RGB palette

[doublex]

This image: dancing.zip

Testcase:

import PIL.Image
with PIL.Image.open('dancing.webp') as img:
    assert img.mode == 'RGB'
    frames = []
    for frame in PIL.ImageSequence.Iterator(img):
        frame = frame.resize( img.size, PIL.Image.BICUBIC )
        assert frame.mode == 'RGB'
        frames.append( frame )
    frames[0].save( 'result.gif', format='GIF', version='GIF89a', save_all=True, append_images=frames[1:], duration=30, loop=0 )

[radarhere]

I've created PR #6792 to resolve this.