`XMLParserMixin.namespaces` versus `Namespace.supported_namespaces`

[masklinn]

These information seem to be largely redundant (mappings of namespace URIs to prefixes), and only the former seems to actually be used in the code (to normalise namespaces / prefixes).

Both seem to date back from the great shuffling around of 2015 so it seems unlikely but does anyone remember why and what? Are the dicts in Namespace just informative? Or was namespaces supposed to be built from them, but turns out Python's inheritance does not support that OOTB which makes the supported_namespaces not super useful in the end?

[kurtmckee]

@masklinn Thanks for bringing this up! I'll have to take a look at this. It might be a mistake or it might have been deliberate. I unfortunately don't recall off the top of my head...