fix failing cnn and hashing tests due to use of pytest.warns(None)

[omgold]

Tests fail on Python 3.11:

tests/test_cnn.py:894: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = WarningsChecker(record=True), expected_warning = None, match_expr = None

    def __init__(
        self,
        expected_warning: Union[Type[Warning], Tuple[Type[Warning], ...]] = Warning,
        match_expr: Optional[Union[str, Pattern[str]]] = None,
        *,
        _ispytest: bool = False,
    ) -> None:
        check_ispytest(_ispytest)
        super().__init__(_ispytest=True)
    
        msg = "exceptions must be derived from Warning, not %s"
        if isinstance(expected_warning, tuple):
            for exc in expected_warning:
                if not issubclass(exc, Warning):
                    raise TypeError(msg % type(exc))
            expected_warning_tup = expected_warning
        elif isinstance(expected_warning, type) and issubclass(
            expected_warning, Warning
        ):
            expected_warning_tup = (expected_warning,)
        else:
>           raise TypeError(msg % type(expected_warning))
E           TypeError: exceptions must be derived from Warning, not <class 'NoneType'>

/usr/lib/python3.11/site-packages/_pytest/recwarn.py:285: TypeError

The reason seems to be that with pytest.warns(None): is not a working method to check for that warnings are emitted with recent versions of pytest.

This fixes it for me.