Backport "new plan: just remove typecasts (closes python#20374)"

See python/cpython@0ac0ead

Author: benjaminp

Modules/readline.c

@@ -850,7 +850,7 @@ on_completion(const char *text, int state)
  * before calling the normal completer */
 
 static char **
-flex_complete(char *text, int start, int end)
+flex_complete(const char *text, int start, int end)
 {
 #ifdef HAVE_RL_COMPLETION_APPEND_CHARACTER
     rl_completion_append_character ='\0';
@@ -898,12 +898,12 @@ setup_readline(void)
     rl_bind_key_in_map ('\t', rl_complete, emacs_meta_keymap);
     rl_bind_key_in_map ('\033', rl_complete, emacs_meta_keymap);
     /* Set our hook functions */
-    rl_startup_hook = (rl_hook_func_t *)on_startup_hook;
+    rl_startup_hook = on_startup_hook;
 #ifdef HAVE_RL_PRE_INPUT_HOOK
-    rl_pre_input_hook = (rl_hook_func_t *)on_pre_input_hook;
+    rl_pre_input_hook = on_pre_input_hook;
 #endif
     /* Set our completion function */
-    rl_attempted_completion_function = (rl_completion_func_t *)flex_complete;
+    rl_attempted_completion_function = flex_complete;
     /* Set Python word break characters */
     completer_word_break_characters =
         rl_completer_word_break_characters =