In #25572, we defined typedefs for commonly used Cython types throughout the codebase. Running grep -rl "unsigned char" ./sklearn, I found the following files contain unsigned char, which could be replaced with uint8_t from https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/utils/_typedefs.pxd.

• ./sklearn/metrics/_pairwise_distances_reduction/_radius_neighbors_classmode.pyx.tp
• ./sklearn/ensemble/_hist_gradient_boosting/common.pxd
• ./sklearn/ensemble/_hist_gradient_boosting/_bitset.pyx
• ./sklearn/ensemble/_hist_gradient_boosting/_predictor.pyx
• ./sklearn/ensemble/_hist_gradient_boosting/splitting.pyx
• ./sklearn/ensemble/_hist_gradient_boosting/histogram.pyx
• ./sklearn/ensemble/_hist_gradient_boosting/_binning.pyx
• ./sklearn/ensemble/_hist_gradient_boosting/_bitset.pxd
• ./sklearn/linear_model/_sgd_fast.pyx.tp

Is there any reason to leave these as unsigned char vs replacing them with the uint8_t to consolidate our types?

To address this issue, it would be best to open up a PR one-by-one and implement the said change and cimport statement, and then link to the issue here.