bpo-38156: Fix compiler warning in PyOS_StdioReadline() (GH-21721)

incr cannot be larger than INT_MAX: downcast to int explicitly.
(cherry picked from commit bde48fd)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Parser/myreadline.c

@@ -318,7 +318,7 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
             return NULL;
         }
         p = pr;
-        int err = my_fgets(tstate, p + n, incr, sys_stdin);
+        int err = my_fgets(tstate, p + n, (int)incr, sys_stdin);
         if (err == 1) {
             // Interrupt
             PyMem_RawFree(p);