[MNT] `mypy` fixes

After #1637 is merged, run the updated Mypy version and review the listed examples and tests modules for reported errors. Address every checked item, including the tests directory after its global skip is removed. Done means the listed modules pass Mypy without skips or reported errors.

Once #1637 is merged:

Fixes required due to Mypy version upgrade (v1.13.0 to v1.19.1)

These files were functional under older Mypy versions but now report errors.

[ ] examples._external_or_deprecated.upload_amlb_flows_and_runs
[ ] examples._external_or_deprecated.run_setup_tutorial
[ ] examples._external_or_deprecated.flows_and_runs_tutorial
[ ] examples._external_or_deprecated.flow_id_tutorial
[ ] examples._external_or_deprecated.fetch_runtimes_tutorial
[ ] examples._external_or_deprecated.benchmark_with_optunahub
[ ] examples._external_or_deprecated.2018_neurips_perrone_example
[ ] examples._external_or_deprecated.2018_ida_strang_example
[ ] examples._external_or_deprecated.2015_neurips_feurer_example
[ ] examples.Basics.simple_tasks_tutorial
[ ] examples.Basics.simple_suites_tutorial
[ ] examples.Basics.simple_flows_and_runs_tutorial
[ ] examples.Basics.simple_datasets_tutorial
[ ] examples.Advanced.task_manual_iteration_tutorial
[ ] examples.Advanced.study_tutorial
[ ] examples.Advanced.fetch_evaluations_tutorial
[ ] examples.Advanced.datasets_tutorial
[ ] examples.Advanced.create_upload_tutorial

Fixes required for the tests directory

These files need to be addressed after removing the global skip on the tests directory.

[ ] tests.conftest
[ ] tests.test_datasets.test_dataset
[ ] tests.test_datasets.test_dataset_functions
[ ] tests.test_evaluations.test_evaluation_functions
[ ] tests.test_evaluations.test_evaluations_example
[ ] tests.test_extensions.test_functions
[ ] tests.test_flows.dummy_learn.dummy_forest
[ ] tests.test_flows.test_flow
[ ] tests.test_flows.test_flow_functions
[ ] tests.test_openml.test_api_calls
[ ] tests.test_openml.test_cli
[ ] tests.test_openml.test_config
[ ] tests.test_openml.test_openml
[ ] tests.test_runs.test_run
[ ] tests.test_runs.test_run_functions
[ ] tests.test_runs.test_trace
[ ] tests.test_setups.test_setup_functions
[ ] tests.test_study.test_study_functions
[ ] tests.test_tasks.test_classification_task
[ ] tests.test_tasks.test_clustering_task
[ ] tests.test_tasks.test_learning_curve_task
[ ] tests.test_tasks.test_regression_task
[ ] tests.test_tasks.test_split
[ ] tests.test_tasks.test_supervised_task
[ ] tests.test_tasks.test_task
[ ] tests.test_tasks.test_task_functions
[ ] tests.test_tasks.test_task_methods
[ ] tests.test_utils.test_utils