Do not read layers immediately when opening PSD images

[radarhere]

When opening a PSD image, this PR skips reading and parsing the PSD layers until the user calls im.layers, im.n_frames or im.is_animated.

This will improve performance, and also allow users who just want to open() an image without seeking to workaround any errors that might be raised when parsing layers, such as in #8034

[hugovk]

👍

main:

❯ python -m timeit -s "from PIL import Image" "Image.open('Tests/images/hopper.psd')"
2000 loops, best of 5: 166 usec per loop

❯ python -m timeit -s "from PIL import Image" "im = Image.open('Tests/images/hopper.psd'); im.layers"
2000 loops, best of 5: 164 usec per loop

PR:

❯ python -m timeit -s "from PIL import Image" "Image.open('Tests/images/hopper.psd')"
2000 loops, best of 5: 101 usec per loop

❯ python -m timeit -s "from PIL import Image" "im = Image.open('Tests/images/hopper.psd'); im.layers"
2000 loops, best of 5: 164 usec per loop

(These runs were actually the third ones, to make sure everything had imported properly and avoid delays after an initial build.)