Use decorator style properties

[lianghai]

Problem

Some recent changes (eg, #189) have created difficulties for Pylance/Pyright to infer that Font.kerning is of type Kerning:

See also a non-property for comparison:

Is it because that the property class isn’t generic? I tried to look into issue tickets in the Pylance and Pyright projects but haven’t found anything helpful.

Potential solution

Seems it works alright if we write the properties (eg, .kerning and .lib) in the decorator style:

To be clear, I don’t really understand type checkers enough to tell if the decorator method is an arbitrary workaround or a long term solution, and I certainly don’t want to make the maintainers to randomly patch ufoLib2 only for some specific type checkers’ temporary limitations…

[madig]

I have not looked into this too deeply, but I think Pylance and others have trouble with attrs doing its own thing. Adding a Kerning annotation to the kerning property fixes the type but leads to mistyping error. Feel free to search https://github.com/microsoft/pylance-release/issues and https://github.com/microsoft/pylance-release/discussions for pointers.

I forgot why we do the kerning = property(_get_kerning, _set_kerning) line... maybe your fix is enough. Will look at this at some point.