style: upgrade to ruff v0.8.4 and fix type hints

[eonu]

Upgrade to ruff v0.8.4 and fix type hints

• Upgrade to ruff v0.8.4.
• Basic type hint fixes (remove typing.Self and annotations for self/cls.

Checklist

• I have added new tests (if necessary).
• I have ensured that tests and coverage are passing on CI.
• I have updated any relevant documentation (if necessary).
• I have used a descriptive pull request title.

[coveralls]

Pull Request Test Coverage Report for Build 82787c24-d0c3-4528-893e-66ef3cf9dc50

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 108 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.05%) to 82.569%

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Files with Coverage Reduction	New Missed Lines	%
models/hmm/variants/categorical.py	4	84.21%
models/knn/base.py	5	94.12%
_internal/_hmm/topologies.py	6	82.98%
datasets/base.py	7	86.55%
models/base.py	7	83.72%
models/knn/classifier.py	7	91.86%
models/hmm/variants/gaussian_mixture.py	8	74.55%
models/hmm/classifier.py	10	91.13%
preprocessing/transforms.py	11	76.47%
model_selection/_validation.py	15	56.48%

Totals
Change from base Build 9bd96fe7-c362-4f83-b701-7fe6550b1a94:	0.05%
Covered Lines:	1170
Relevant Lines:	1417

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💛 - Coveralls