gh-126662: Naming consistency for shutil._ntuple_diskusage

[tungol]

This MR updates shutil._ntuple_diskusage so it's named consistently.

• Issue: Naming consistency for various types #126662

[tungol]

Marking this one as draft to consider: this is like #126721 in that it's a named tuple returned directly to end users, and _ntuple_diskusage is a weird looking name, so maybe this isn't the best solution to achieve consistency.