gh-133143: Use _Py_ID for the other literals in sys (GH-138698)

Follow-up refactoring after GH-133143, where we use `_Py_ID` for "big" and "little"
in `abi_info.byteorder`.
This uses `_Py_ID` for `sys.byteorder`, but also `float_repr_style` and a module name.