Using :func:`pytest.approx` in a boolean context now raises an error hinting at the proper usage.

It is apparently common for users to mistakenly use ``pytest.approx`` like this:

.. code-block:: python

    assert pytest.approx(actual, expected)

While the correct usage is:

.. code-block:: python

    assert actual == pytest.approx(expected)

The new error message helps catch those mistakes.