Prevent setting non-existent struct fields (ctypesgen issue)

[mara004]

Setting a wrong struct field silently passes. Think about a utility that would prevent this.
See bblanchon/pdfium-binaries#105 (comment) for context.

[mara004]

Incidentally, an independent SO thread on this topic has been raised very recently: https://stackoverflow.com/questions/76887146/prevent-assignment-of-non-existent-field-in-ctypes-structure

[mara004]

Turns out this is a ctypesgen issue, see ctypesgen/ctypesgen#183 (comment).
__slots__ can be used to prevent non-existent fields, but this only works if they're actually defined in the class body, not when added afterwards as ctypesgen does ATM.

[mara004]

Done in our fork. Fix not upstreamed yet, though.