don't leak file descriptors in test

if no f_in and f_out arguments are given, _ReadlineWrapper calls os.dup
on fd 0 and 1. Instead, just pass None, because the fds aren't needed
anyway in this test

Lib/test/test_pyrepl/test_pyrepl.py

@@ -519,7 +519,7 @@ def test_basic(self):
 
     def test_get_line_buffer_returns_str(self):
         reader = self.prepare_reader(code_to_events("\n"))
-        wrapper = _ReadlineWrapper(reader=reader)
+        wrapper = _ReadlineWrapper(f_in=None, f_out=None, reader=reader)
         self.assertIs(type(wrapper.get_line_buffer()), str)
 
     def test_multiline_edit(self):