Fix incompatibility for CssProvider.load_from_data() (GTK 4.10)

Merged Arjan Molenaar requested to merge amolenaar/pygobject:gtk-4-10-css-provider into master

In GTK 4.8, bytes are expected, in GTK 4.10, you can provider a string, with a length.

This patch still allows bytes for backwards compatibility, and add support for strings in GTK 4.8 and before.

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