Doc: sys.__unraisablehook__ and bytearray.hex separators are new in 3.8 (GH-17884)

Minor fix in documentation:

- `sys.__unraisablehook__` is new in version 3.8
- Optional `sep` and `bytes_per_sep` parameters for `bytearray.hex` is also supported in Python 3.8 (just like `bytes.hex`)