Remove scikit-image from dependencies

[IRDonch]

Summary

The sole usage of it was removed in ce0a122.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[zhiltsov-max]

Do we need a version restriction?

[IRDonch]

Surprisingly, no, because Datumaro only uses a single SciPy function, scipy.linalg.orth, which was added before Python 3 even existed. :-)