tests/test_open_file: restore sys.stdin.

Fixes:

    $ python3 -m pytest
    ...
    FAILED tests/test_open_file.py::test_open_stdin::test - AttributeError: 'DontReadFromInput' object has no attribute 'readable'

Work-around for: pytest-dev/pytest#10150

tests/test_open_file.py

@@ -19,6 +19,8 @@
 # SOFTWARE.
 
 import os
+import sys
+import unittest
 
 from tests.tools import (
     assert_equal,
@@ -40,7 +42,8 @@ def test_open_real_file():
 
 @testcase
 def test_open_stdin():
-    t('-')
+    with unittest.mock.patch('sys.stdin', sys.__stdin__):
+        t('-')
 
 del testcase