fix GenericPlainRegistry getattr type

[theodotk]

• Closes mypy error: ureg.m annotation is expected to be Quantity instead of Unit #1946 and Type error in Quantity constructor #1804
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate -- no need to document, it's a bugfix for type
• Added an entry to the CHANGES file

I'm aware that there exists PR #1948 , but it's not been merged for a while

[andrewgsavage]

thanks, those tests make it clear what's intended.

[codspeed-hq]

CodSpeed Performance Report

Merging #2045 will not alter performance

_{Comparing theodotk:master (e0375e9) with master (0b0ae61)}

Summary

✅ 437 untouched benchmarks

[theodotk]

@andrewgsavage @hgrecco could you please make a release?