subtyping PyVarObject (e.g. bytes/tuple)

[scoder]

The problem with a PyVarObject (such as bytes or tuple) is that its struct is of variable length that is determined at instance creation time. When Cython generates subclass code, it expects to be able to add fields directly behind the compile time struct, which thus end up in the variably allocated memory area.

[scoder]

This also impacts the tp_dictoffset slot, so it has an impact on PyVarObject subtypes that use an instance __dict__.