bpo-34403: Always implement _Py_GetForceASCII() (GH-10235)

Compilation fails on macOS because _Py_GetForceASCII() wasn't define:
always implement implement (default implementation: just return 0).

Python/fileutils.c

@@ -246,6 +246,12 @@ encode_ascii(const wchar_t *text, char **str,
     *str = result;
     return 0;
 }
+#else
+int
+_Py_GetForceASCII(void)
+{
+    return 0;
+}
 #endif   /* !defined(__APPLE__) && !defined(__ANDROID__) && !defined(MS_WINDOWS) */