sys.flags.int_max_str_digits is not -1

## Summary

sys.flags.int_max_str_digits now returns sys.get_int_max_str_digits().

Introduced from https://github.com/RustPython/RustPython/pull/5127/files#diff-128fff16ea63b541718887f5c714e0c1a357414fe1146b4893ef699e8853e3b0R953

Related RustPython code is `vm/src/stdlib.sys.rs`  `Flags::int_max_str_digits`.

Not sure yet if we need to change the value of Settings or sys::Flags.
