[Datumaro] Use attrs for model declaration

[zhiltsov-max]

Motivation and context

This patch refactors Datumaro definitions using datatypes to simplify and reduce code.

• Refactored annotation types
• Annotations can be copied and printed
• DatasetItems can be copied and printed
• LabelRemap transform is no more in-place (as intended)
• Fixed few bandit warnings

How has this been tested?

Unit tests

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6835

• 113 of 115 (98.26%) changed or added relevant lines in 7 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.08%) to 68.848%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/components/extractor.py	89	90	98.89%
datumaro/datumaro/util/attrs_util.py	16	17	94.12%

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/components/extractor.py	1	91.99%
datumaro/datumaro/components/launcher.py	1	80.36%
datumaro/datumaro/plugins/coco_format/converter.py	2	89.24%

Totals
Change from base Build 6817:	-0.08%
Covered Lines:	11072
Relevant Lines:	15585

---

💛 - Coveralls

[nmanovic]

attrs looks nice!