bpo-14916: use specified tokenizer fd for file input (GH-31006)

@pablogsal, sorry i failed to rebase to main, so i recreated #22190 (comment)

> PyRun_InteractiveOne\*() functions allow to explicitily set fd instead of stdin.
but stdin was hardcoded in readline call.

> This patch does not fix target file for prompt unlike original bpo one : prompt fd is unrelated to tokenizer source which could be read only. It is more of a bugfix regarding the docs :  actual documentation say "prompt the user" so one would expect prompt to go on stdout not a file for both PyRun_InteractiveOne\*() and PyRun_InteractiveLoop\*().

Automerge-Triggered-By: GH:pablogsal

Author: pmp-p

Misc/NEWS.d/next/C API/2020-09-11-02-50-41.bpo-14916.QN1Y03.rst

@@ -0,0 +1 @@
+Fixed bug in the tokenizer that prevented ``PyRun_InteractiveOne`` from parsing from the provided FD.

Parser/tokenizer.c

@@ -855,7 +855,7 @@ tok_underflow_interactive(struct tok_state *tok) {
         tok->done = E_INTERACT_STOP;
         return 1;
     }
-    char *newtok = PyOS_Readline(stdin, stdout, tok->prompt);
+    char *newtok = PyOS_Readline(tok->fp ? tok->fp : stdin, stdout, tok->prompt);
     if (newtok != NULL) {
         char *translated = translate_newlines(newtok, 0, tok);
         PyMem_Free(newtok);