Update the signature of PyFile_WriteString().

Doc/api/concrete.tex

@@ -1932,7 +1932,7 @@ implementation detail and may change in future releases of Python.
   failure; the appropriate exception will be set.
 \end{cfuncdesc}
 
-\begin{cfuncdesc}{int}{PyFile_WriteString}{char *s, PyFileObject *p}
+\begin{cfuncdesc}{int}{PyFile_WriteString}{const char *s, PyFileObject *p}
   Writes string \var{s} to file object \var{p}.  Returns \code{0} on
   success or \code{-1} on failure; the appropriate exception will be
   set.

Doc/api/refcounts.dat

@@ -314,7 +314,7 @@ PyFile_WriteObject:PyFileObject*:p:0:
 PyFile_WriteObject:int:flags::
 
 PyFile_WriteString:int:::
-PyFile_WriteString:char*:s::
+PyFile_WriteString:const char*:s::
 PyFile_WriteString:PyFileObject*:p:0:
 PyFile_WriteString:int:flags::