Since this is a float comparison, it would be wiser to use `assert_array_almost_equal` here.

Also, this is a good test, but it by no means exhaustive. I'd like to, in addition to this test, see that the composition of `PowerNorm(2.3)` and `PowerNorm(2.3).inverse` is the identity map. This will also need the `assert_array_almost_equal` function.