gh-93937: PyOS_StdioReadline() uses PyConfig.legacy_windows_stdio (#94024)

On Windows, PyOS_StdioReadline() now gets
PyConfig.legacy_windows_stdio from _PyOS_ReadlineTState, rather than
using the deprecated global Py_LegacyWindowsStdioFlag variable.

Fix also a compiler warning in Py_SetStandardStreamEncoding().

Author: vstinner

Diff for: Parser/myreadline.c

@@ -247,7 +247,8 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
     assert(tstate != NULL);
 
 #ifdef MS_WINDOWS
-    if (!Py_LegacyWindowsStdioFlag && sys_stdin == stdin) {
+    const PyConfig *config = _PyInterpreterState_GetConfig(tstate->interp);
+    if (!config->legacy_windows_stdio && sys_stdin == stdin) {
         HANDLE hStdIn, hStdErr;
 
         hStdIn = _Py_get_osfhandle_noraise(fileno(sys_stdin));

Diff for: Python/initconfig.c

@@ -543,8 +543,11 @@ Py_SetStandardStreamEncoding(const char *encoding, const char *errors)
     }
 #ifdef MS_WINDOWS
     if (_Py_StandardStreamEncoding) {
+_Py_COMP_DIAG_PUSH
+_Py_COMP_DIAG_IGNORE_DEPR_DECLS
         /* Overriding the stream encoding implies legacy streams */
         Py_LegacyWindowsStdioFlag = 1;
+_Py_COMP_DIAG_POP
     }
 #endif