WeasyPrint using deprecated fontTools API

I fontTools version [4.59.1](https://github.com/fonttools/fonttools/releases/tag/4.59.1) the `fonttools varLib.mutator.instantiateVariableFont` function was deprecated in favour of `fonttools varLib. instancer.instantiateVariableFont`.

WeasyPrint is using the deprecated API [here](https://github.com/Kozea/WeasyPrint/blob/eb47d81dd1dc4f7080bb45fa75e0515cf2c5ad5e/weasyprint/pdf/fonts.py#L12).