bpo-34403: Fix initfsencoding() for ASCII (GH-10233)

* Add _Py_GetForceASCII(): check if Python forces the usage of ASCII
  in Py_DecodeLocale() and Py_EncodeLocale().
* initfsencoding() now uses ASCII if _Py_GetForceASCII() is true.
diff --git a/Include/fileutils.h b/Include/fileutils.h
index c05ff43..d75189a 100644
--- a/Include/fileutils.h
+++ b/Include/fileutils.h
@@ -183,6 +183,10 @@
 
 #endif   /* Py_LIMITED_API */
 
+#ifdef Py_BUILD_CORE
+PyAPI_FUNC(int) _Py_GetForceASCII(void);
+#endif
+
 #ifdef __cplusplus
 }
 #endif