_Py_stat() and _Py_fopen(): avoid PyUnicode_AsWideCharString() on Windows

On Windows, Py_UNICODE is wchar_t, so we can avoid the expensive Py_UNICODE*
=> wchar_t* conversion.
diff --git a/Include/fileutils.h b/Include/fileutils.h
index 9b2d34d..1d8df15 100644
--- a/Include/fileutils.h
+++ b/Include/fileutils.h
@@ -19,7 +19,7 @@
 
 #ifdef HAVE_STAT
 PyAPI_FUNC(int) _Py_stat(
-    PyObject *unicode,
+    PyObject *path,
     struct stat *statbuf);
 #endif
 
@@ -28,7 +28,7 @@
     const wchar_t *mode);
 
 PyAPI_FUNC(FILE*) _Py_fopen(
-    PyObject *unicode,
+    PyObject *path,
     const char *mode);
 
 #ifdef HAVE_READLINK