Allow pytest native tests

[zhiltsov-max]

Summary

Pytest offers lots of interesting features, including fixtures and test marking. Fixtures, specifically, can be used to better manage test resources and speed-up tests.

• Allowed test lookup by name
• Test classes are not required to be inherited from TestCase anymore (though, they can be)
• Tests can be expressed by free functions outside classes
• Dropped unittest support
• datumaro.utils.test_utils.test_save_and_load renamed no to be confused with a test

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

[IRDonch]

I'm not sure this needs to be in the changelog, since it's not a user-facing change. Up to you, though.

[zhiltsov-max]

I'll remove it from release notes, but it looks as important for developers. Need to think about this again.