[BUG] `testdir` fixture usage seems to cause `PytestDeprecationWarning: The TerminalReporter.writer attribute is deprecated, use TerminalReporter._tw instead at your own risk.`

[webknjaz]

While implementing pytest-dev/pytest-forked#34, I've faced this, when writing tests for the plugin:

  $PYTHON_PREFIX/lib/python3.8/site-packages/_pytest/compat.py:333: PytestDeprecationWarning: The TerminalReporter.writer attribute is deprecated, use TerminalReporter._tw instead at your own risk.
  See https://docs.pytest.org/en/latest/deprecations.html#terminalreporter-writer for more information.
    return getattr(object, name, default)

It's funny that the docs even showcase this exact warning in the test report snippet in this section: https://docs.pytest.org/en/stable/writing_plugins.html#testing-plugins.

N.B. I'm currently unable to determine what exactly triggers this because of the dynamic getattr().

[asottile]

dupe of #6936