More Mitsuba Python stubs improvements

[rtabbara]

• Going through pyright output to identity some legitimate issues with the bindings, primarily with repeated bindings.
• For the variant submodules, including them in the top-level namespace so that the relative imports in the stubs are variant agnostic.
• Attempt to express Spectrum and UnpolarizedSpectrum in a variant-agnostic way that still provides some specificity as a templated dr.ArrayBase type
• Generating stubs for native and Python submodules