The CPython uses _Py_dg_dtoa(), which does rounding to nearest with half
to even tie-breaking rule.

If that functions is unavailable, PyOS_double_to_string() fallbacks to
system snprintf().  Since CPython 3.12, build requirements include C11
compiler *and* support for IEEE 754 floating point numbers (Annex F).
This means that FE_TONEAREST macro is available and, per default,
printf-like functions should use same rounding mode as _Py_dg_dtoa().

Co-authored-by: Bénédikt Tran <[email protected]>