Fix broken string in test error message

[hugovk]

Hasn't cropped up because the test is passing :)

Demo

average_diff = 2
max_average_diff = 1
band = "R"

# broken
msg = "average pixel value difference {:.4f} > expected {:.4f} "
"for '{}' band".format(average_diff, max_average_diff, band)

print(msg)

# fixed
msg = (
    "average pixel value difference {:.4f} > expected {:.4f} "
    "for '{}' band".format(average_diff, max_average_diff, band)
)

print(msg)

average pixel value difference {:.4f} > expected {:.4f}
average pixel value difference 2.0000 > expected 1.0000 for 'R' band

[radarhere]

Nice catch

[hugovk]

Found by the new Flake8 3.8.

There's a few other minor things too, but I've not updated those (or pre-commit) yet as Black doesn't like it, so they can wait a bit.

psf/black#195 (comment)