Change PyOS_Readline declaration to match the recent change to myreadline.c

(see Patch 512981).  I changed stdin to sys_stdin in the body of the
function, but did not change stderr to sys_stdout, though I suspect that may
be the correct course.  I don't know the code involved well enough to judge.

Parser/pgenmain.c

@@ -165,7 +165,7 @@ guesstabsize(char *path)
 /* No-nonsense my_readline() for tokenizer.c */
 
 char *
-PyOS_Readline(char *prompt)
+PyOS_Readline(FILE *sys_stdin, FILE *sys_stdout, char *prompt)
 {
 	size_t n = 1000;
 	char *p = PyMem_MALLOC(n);
@@ -173,7 +173,7 @@ PyOS_Readline(char *prompt)
 	if (p == NULL)
 		return NULL;
 	fprintf(stderr, "%s", prompt);
-	q = fgets(p, n, stdin);
+	q = fgets(p, n, sys_stdin);
 	if (q == NULL) {
 		*p = '\0';
 		return p;