Migrate to pyproject.toml

[matthias-baer]

Somehow, the API for the unwrap_phase function in skimage has changed, so I had to replace seed by rng.

One of the tests didn't run with the default settings of atol=1.E-8. Changed it to atol=2.E-6 to make it pass, but maybe you have a look at it.

[paulmueller]

The failing tests might be a Windows thing. On Linux everything still passes 👍.
I incremented the minimal scikit-image version to 0.21.0 due to the rng keyword argument.