Update dependencies and fix imports in conftest.py

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Fix breaking tests: In conftest.py, the testing mock utility from moto import mock_s3 has been updated to mock_aws. as mock_s3 is been deprecated and breaking tests.
Dependency Update: In setup.cfg, the dependency pytest-black has been replaced with pytest-black-ng, and the Sphinx version constraint has been relaxed from a fixed version ==4.2.0 to >=4.2.0.
Code Modernization: Removal of from future import. This indicates a move away from Python 2 compatibility layers, transitioning to Python 3 as the minimum supported version.

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inveniosoftware / invenio-s3