I need to inject some XMP metadata into an image to be written to disk as (big)TIFF.

Some related functionality seems to be documented in the libvips header documentation but I'm missing a clear path on how to insert a simple property in pyvips.

E.g. if I try to use img.set('xmp:copyright', copy_str) I get an error that I need to define the gtype for the property, but if I want to use set_type I am not sure which value I shall use for my type (str).

I also see a VIPS-META-XMP-NAME constant is defined in libvips but It's not clear to me how to use it or how to access it in pyvips.

Any help is appreciated.
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