i128 / u128: serialisation returns Err with no documentation of the limitation #112

Summary

Apologies for taking up your time - I do hope this is useful rather than a bother.

While working on an extended test suite for pythonize (see PRs #108-#110), I noticed that attempting to serialise a Rust i128 or u128 value produces an Err at runtime, but this limitation does not appear to be documented anywhere in the README, API docs, or inline comments.

Reproduction

Python::attach(|py| {
    let val: i128 = 1;
    let result = pythonize(py, &val); // returns Err
    println!("{:?}", result);
});

Expected behaviour

Either a clear error at serialisation time is fine (Python's int can represent arbitrary integers, so a conversion could in principle be supported), or -- if the limitation is intentional -- a note in the README type-mapping table would save users a rather puzzling debugging session.

Suggested addition (no code required)

A single line in the README type table noting that i128 / u128 are not currently supported would be perfectly sufficient. I would be very happy to include such a note in one of the open test PRs if that would be convenient.

Many thanks for maintaining this crate -- it is a genuinely useful piece of work.