gh-35458: remove old deprecated properties in set_partition + pep8 cleanup
    
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### 📚 Description

This removes two old deprecated properties in `set_partition.py`,
replaced by methods in #25462 and #25865.

Also performs a pycodestyle cleanup of the modified file.

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URL: #35458
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager