tests/test_open_file: restore sys.stdin.
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: https://github.com/pytest-dev/pytest/issues/10150

jwilk/anorack,jwilk/anorack