[FIX] Adapt tests to pandas 2.1 and fix deprecations

[PrimozGodec]

Description of changes

• CI - Stop testing Python 3.8 since it isn't recommended to support it anymore, according to NEP 29. Pandas, Numpy, and Scipy dropped support already (after reading skip in the second point of this PR, it is unnecessary to drop those tests anymore, but I would go further with this anyway).
• Re-add skip to pandas test that was removed as part of [FIX] Adapt to newly released pandas 2.1 #6559 (since, at some point, we will probably run the oldest supported workflow with Pandas 2.0.*). It was updating the skipp that was still present and a minor update of the test.
• Handle some deprecations in the Pandas module (is_categorical_dtype and setting value by index without iloc).
• Reupdate import from [FIX] Adapt to newly released pandas 2.1 #6559

Includes

• Code changes
• Tests
• Documentation