Add What's new entry

Author: pablogsal

Doc/whatsnew/3.8.rst

@@ -1042,6 +1042,11 @@ Build and C API Changes
   allocation or deallocation may need to be adjusted.
   (Contributed by Eddie Elizondo in :issue:`35810`.)
 
+* The new function :c:func:`PyCode_NewWithPosOnlyArgs` allows to create
+  code objects like :c:func:`PyCode_New`, but with an extra *posonlyargcount*
+  parameter for indicating the number of positional-only arguments.
+  (Contributed by Pablo Galindo in :issue:`37221`.)
+
 
 Deprecated
 ==========