Add tabulate to requirements-core.txt

[vinnamkim]

Summary

• Resolve Tabulate is imported, but not presented in requirements #1065
• tabulate was introduced in CLI update from diff to compare & add TableComparator #1012, but not added to requirements-core.txt
• The reason why we haven't found it is because tabulate is dependency of dvc, so that there has been no problem if we install datumaro[default].

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

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  Feel free to contact the maintainers if that's a concern.
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# Copyright (C) 2023 Intel Corporation
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