using _PyIO_ConvertSsize_t sounds great.

with regard to tests for accepting integer types in other classes,
there aren't a lot of them, so I guess it is not always tested.
still, it is tested in (excluding tests of the actual feature of
treating integer types as ints): test_bytes, test_cmath, test_int,
test_math, test_range, test_re, test_slice, test_struct,
test_unicode and test_weakref.