Test:
`warnings.warn()` expects that its first argument is a `str` or a
`Warning`, but since 9454fc38d3
`pytest.warns()` no longer allows `Warning` instances unless the first
argument the `Warning` was initialized with is a `str`. Furthermore, if
the `Warning` was created without arguments then `pytest.warns()` raises
an unexpected `IndexError`. The new tests reveal the problem.
Fix:
`pytest.warns()` now allows using `warnings.warn()` with a `Warning`
instance, as is required by Python, with one exception. If the warning
used is a `UserWarning` that was created by passing it arguments and the
first argument was not a `str` then `pytest.raises()` still considers
that an error. This is because if an invalid type was used in
`warnings.warn()` then Python creates a `UserWarning` anyways and it
becomes impossible for `pytest` to figure out if that was done
automatically or not.
[ran: rebased on previous commit]
0475b1c925