Support for python subinterpreters when using upb

[lklein53]

What language does this apply to?
Python

Describe the problem you are trying to solve.
We noticed that if we enable subinterpreters in python the underlying protobuf implementation switches from upb to python. This has some performance impact that we would like to avoid.

Describe the solution you'd like
Be able to use the upb protobuf implementation in python subinterpreters.

Additional context
We have seen that there is some ongoing work to get free threading python support into the library. Is there already similar work ongoing to support subinterpreters? We would also be willing to contribute if subinterpretert support is a feature that is considered worth adding to the library.

thanks and best regards

[jguamie]

We'd be happy to accept contributions for subinterpreter. Can you please provide a design on how you intend to do this prior to writing the code implementation?