Update function renamed in skimage 0.16.

[sebastic]

test_psnr fails with skimage 0.18 because skimage.measure.compare_psnr was renamed to skimage.metrics.peak_signal_noise_ratio in 0.16.

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ERROR: test_psnr (tests.test_jp2k.TestJp2k_write)
SCENARIO:  Four peak signal-to-noise ratio values are supplied, the
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Traceback (most recent call last):
  File "/<<PKGBUILDDIR>>/tests/test_jp2k.py", line 1309, in test_psnr
    psnr = [
  File "/<<PKGBUILDDIR>>/tests/test_jp2k.py", line 1310, in <listcomp>
    skimage.measure.compare_psnr(skimage.data.camera(), d[j])
AttributeError: module 'skimage.measure' has no attribute 'compare_psnr'

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Ran 468 tests in 44.506s