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`)
(cherry picked from commit 0edc2c7678266c39a7ceb2df885cb050f887e32b)

Co-authored-by: Saiyang Gou <gousaiyang@163.com>
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index e1d93f8..d3473de 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -343,6 +343,8 @@
    .. versionadded:: 3.7
       __breakpointhook__
 
+   .. versionadded:: 3.8
+      __unraisablehook__
 
 .. function:: exc_info()