explicitly implement dtype on numpy quantities

[keewis]

This is important because runtime-checkable protocols require the attribute to be in the class dict from 3.12 onwards, bypassing __getattr__.

• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Added an entry to the CHANGES file

[codspeed-hq]

CodSpeed Performance Report

Merging #1922 will not alter performance

_{Comparing keewis:dtype (7f5db09) with master (0f24b6f)}

Summary

✅ 439 untouched benchmarks