``Testdir.run().parseoutcomes()`` now always returns the parsed nouns in plural form.

Originally ``parseoutcomes()`` would always returns the nouns in plural form, but a change
meant to improve the terminal summary by using singular form single items (``1 warning`` or ``1 error``)
caused an unintended regression by changing the keys returned by ``parseoutcomes()``.

Now the API guarantees to always return the plural form, so calls like this:

.. code-block:: python

    result = testdir.runpytest()
    result.assert_outcomes(error=1)

Need to be changed to:


.. code-block:: python

    result = testdir.runpytest()
    result.assert_outcomes(errors=1)