Comment 1 for bug 1713707

Thanks for the report.

You didn't say what kind of error you got, but since you already quoted the relevant section of the docs, it's probably a crash.
Subinterpreters are barely supported by any extension modules in CPython, and not at all (currently) by Cython. Probably doable in general, but then, libxml2/libxslt also have some corners where they require global state (as opposed to per-thread or per-subinterpreter state).

In short: difficult, and won't happen any time soon.