python-tox fails to build with Python 3.14.0a6.

_ test_str_convert_nok[3-Union-TypeError-3 cannot cast to typing.Union\\[str, int\\]] _
[gw0] linux -- Python 3.14.0 /usr/bin/python3

raw = '3', of_type = str | int
msg = '3 cannot cast to typing.Union\\[str, int\\]'
exc_type = <class 'TypeError'>

    @pytest.mark.parametrize(
        ("raw", "of_type", "exc_type", "msg"),
        [
            ("a", TypeVar, TypeError, r"a cannot cast to .*typing.TypeVar.*"),
            ("3", Literal["1", "2"], ValueError, r"3 must be one of \('1', '2'\)"),
            ("3", Union[str, int], TypeError, r"3 cannot cast to typing.Union\[str, int\]"),
            ("", Command, ValueError, r"attempting to parse '' into a command failed"),
        ],
    )
    def test_str_convert_nok(raw: str, of_type: type[Any], msg: str, exc_type: type[Exception]) -> None:
        with pytest.raises(exc_type, match=msg):
>           StrConvert().to(raw, of_type, None)

exc_type   = <class 'TypeError'>
msg        = '3 cannot cast to typing.Union\\[str, int\\]'
of_type    = str | int
raw        = '3'

tests/config/loader/test_str_convert.py:86: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../BUILDROOT/usr/lib/python3.14/site-packages/tox/config/loader/convert.py:33: in to
    return self._to_typing(raw, of_type, factory)
        factory    = None
        from_module = 'typing'
        of_type    = str | int
        raw        = '3'
        self       = <tox.config.loader.str_convert.StrConvert object at 0x7f2515eb1a90>
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tox.config.loader.str_convert.StrConvert object at 0x7f2515eb1a90>
raw = '3', of_type = str | int, factory = None

    def _to_typing(self, raw: T, of_type: type[V], factory: Factory[V]) -> V:  # noqa: C901
        origin = getattr(of_type, "__origin__", of_type.__class__)
        result: Any = _NO_MAPPING
        if origin in {list, List}:
            entry_type = of_type.__args__[0]  # type: ignore[attr-defined]
            result = [self.to(i, entry_type, factory) for i in self.to_list(raw, entry_type)]
            if isclass(entry_type) and issubclass(entry_type, Command):
                result = [i for i in result if i is not None]
        elif origin in {set, Set}:
            entry_type = of_type.__args__[0]  # type: ignore[attr-defined]
            result = {self.to(i, entry_type, factory) for i in self.to_set(raw, entry_type)}
        elif origin in {dict, Dict}:
            key_type, value_type = of_type.__args__[0], of_type.__args__[1]  # type: ignore[attr-defined]
            result = OrderedDict(
                (self.to(k, key_type, factory), self.to(v, value_type, factory))
                for k, v in self.to_dict(raw, (key_type, value_type))
            )
        elif origin == Union:  # handle Optional values
            args: list[type[Any]] = of_type.__args__  # type: ignore[attr-defined]
            none = type(None)
            if len(args) == 2 and none in args:  # noqa: PLR2004
                if isinstance(raw, str):
                    raw = raw.strip()  # type: ignore[assignment]
                if not raw:
                    result = None
                else:
                    new_type = next(i for i in args if i != none)  # pragma: no cover
                    result = self.to(raw, new_type, factory)
        elif origin in {Literal, type(Literal)}:
            choice = of_type.__args__  # type: ignore[attr-defined]
            if raw not in choice:
                msg = f"{raw} must be one of {choice}"
                raise ValueError(msg)
            result = raw
        if result is not _NO_MAPPING:
            return cast("V", result)
        msg = f"{raw} cannot cast to {of_type!r}"
>       raise TypeError(msg)
E       TypeError: 3 cannot cast to str | int

args       = (<class 'str'>, <class 'int'>)
factory    = None
msg        = '3 cannot cast to str | int'
none       = <class 'NoneType'>
of_type    = str | int
origin     = <class 'typing.Union'>
raw        = '3'
result     = <object object at 0x7f251d779830>
self       = <tox.config.loader.str_convert.StrConvert object at 0x7f2515eb1a90>

../BUILDROOT/usr/lib/python3.14/site-packages/tox/config/loader/convert.py:92: TypeError

During handling of the above exception, another exception occurred:

raw = '3', of_type = str | int
msg = '3 cannot cast to typing.Union\\[str, int\\]'
exc_type = <class 'TypeError'>

    @pytest.mark.parametrize(
        ("raw", "of_type", "exc_type", "msg"),
        [
            ("a", TypeVar, TypeError, r"a cannot cast to .*typing.TypeVar.*"),
            ("3", Literal["1", "2"], ValueError, r"3 must be one of \('1', '2'\)"),
            ("3", Union[str, int], TypeError, r"3 cannot cast to typing.Union\[str, int\]"),
            ("", Command, ValueError, r"attempting to parse '' into a command failed"),
        ],
    )
    def test_str_convert_nok(raw: str, of_type: type[Any], msg: str, exc_type: type[Exception]) -> None:
>       with pytest.raises(exc_type, match=msg):
E       AssertionError: Regex pattern did not match.
E        Regex: '3 cannot cast to typing.Union\\[str, int\\]'
E        Input: '3 cannot cast to str | int'

exc_type   = <class 'TypeError'>
msg        = '3 cannot cast to typing.Union\\[str, int\\]'
of_type    = str | int
raw        = '3'

https://docs.python.org/3.14/whatsnew/3.14.html

For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.14/fedora-rawhide-x86_64/08783548-python-tox/

For all our attempts to build python-tox with Python 3.14, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.14/package/python-tox/

Testing and mass rebuild of packages is happening in copr.
You can follow these instructions to test locally in mock if your package builds with Python 3.14:
https://copr.fedorainfracloud.org/coprs/g/python/python3.14/

Let us know here if you have any questions.

Python 3.14 is planned to be included in Fedora 43.
To make that update smoother, we're building Fedora packages with all pre-releases of Python 3.14.
A build failure prevents us from testing all dependent packages (transitive [Build]Requires),
so if this package is required a lot, it's important for us to get it fixed soon.

We'd appreciate help from the people who know this package best,
but if you don't want to work on this now, let us know so we can try to work around it on our side.