Passing the ``message`` parameter of ``pytest.raises`` now issues a ``DeprecationWarning``.

It is a common mistake to think this parameter will match the exception message, while in fact
it only serves to provide a custom message in case the ``pytest.raises`` check fails. To avoid this
mistake and because it is believed to be little used, pytest is deprecating it without providing
an alternative for the moment.

If you have concerns about this, please comment on `issue #3974 <https://github.com/pytest-dev/pytest/issues/3974>`__.