[3.8] bpo-37206: Unrepresentable default values no longer represented as None. (GH-13933) (GH-16141)

In ArgumentClinic, value "NULL" should now be used only for unrepresentable default values
(like in the optional third parameter of getattr). "None" should be used if None is accepted
as argument and passing None has the same effect as not passing the argument at all.
(cherry picked from commit 279f44678c8b84a183f9eeb85e0b086228154497)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Objects/clinic/bytesobject.c.h b/Objects/clinic/bytesobject.c.h
index 7e2ce1a..22024ab 100644
--- a/Objects/clinic/bytesobject.c.h
+++ b/Objects/clinic/bytesobject.c.h
@@ -688,7 +688,7 @@
 }
 
 PyDoc_STRVAR(bytes_hex__doc__,
-"hex($self, /, sep=None, bytes_per_sep=1)\n"
+"hex($self, /, sep=<unrepresentable>, bytes_per_sep=1)\n"
 "--\n"
 "\n"
 "Create a str of hexadecimal numbers from a bytes object.\n"
@@ -755,4 +755,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=dbed3c3ff6faa382 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=ca60dfccf8d51e88 input=a9049054013a1b77]*/