myst-nb fails to build with Python 3.15.0b1.

________________ test_custom_convert_multiple_extensions_cache _________________

sphinx_run = <conftest.SphinxFixture object at 0x7f41792e1fd0>
file_regression = <conftest.FileRegression object at 0x7f4173e6c910>
check_nbs = <function check_nbs.<locals>._check_nbs at 0x7f417920bc10>

    @pytest.mark.sphinx_params(
        "custom-formats2.extra.exnt",
        conf={
            "nb_execution_mode": "cache",
            "nb_custom_formats": {".extra.exnt": ["jupytext.reads", {"fmt": "Rmd"}]},
        },
    )
    def test_custom_convert_multiple_extensions_cache(
        sphinx_run, file_regression, check_nbs
    ):
        """The outputs should be populated."""
        sphinx_run.build()
        assert sphinx_run.warnings() == ""
>       regress_nb_doc(file_regression, sphinx_run, check_nbs)

/builddir/build/BUILD/myst-nb-1.4.0-build/MyST-NB-1.4.0/tests/test_execute.py:406: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/builddir/build/BUILD/myst-nb-1.4.0-build/MyST-NB-1.4.0/tests/test_execute.py:38: in regress_nb_doc
    file_regression.check(doctree_string, extension=".xml", encoding="utf-8")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <conftest.FileRegression object at 0x7f4173e6c910>
data = '<document source="custom-formats2" translation_progress="{\'total\': 0, \'translated\': 0}">\n    <section ids="custo...a7452d74c403.png" uri="_build/jupyter_execute/0b6a8ea915d4261d71ec8b0a09154b40bec3fdc3f17723df98aca7452d74c403.png">\n'
kwargs = {'encoding': 'utf-8', 'extension': '.xml'}

    def check(self, data, **kwargs):
>       return self.file_regression.check(self._strip_ignores(data), **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       AssertionError: FILES DIFFER:
E       /tmp/pytest-of-mockbuild/pytest-0/test_custom_convert_multiple_e1/test_custom_convert_multiple_extensions_cache.xml
E       /tmp/pytest-of-mockbuild/pytest-0/test_custom_convert_multiple_e1/test_custom_convert_multiple_extensions_cache.obtained.xml
E       HTML DIFF: /tmp/pytest-of-mockbuild/pytest-0/test_custom_convert_multiple_e1/test_custom_convert_multiple_extensions_cache.obtained.diff.html
E       --- 
E       +++ 
E       @@ -19,4 +19,4 @@
E                                <literal_block classes="output text_plain" language="myst-ansi" xml:space="preserve">
E                                    <Figure size 640x480 with 1 Axes>
E                            <container mime_type="image/png">
E       -                        <image candidates="{'*': '_build/jupyter_execute/321b226a9bda4595da09f9389cea3b97e91b9c089e9bab3feae10e2100fc5c38.png'}" uri="_build/jupyter_execute/321b226a9bda4595da09f9389cea3b97e91b9c089e9bab3feae10e2100fc5c38.png">
E       +                        <image candidates="{'*': '_build/jupyter_execute/0b6a8ea915d4261d71ec8b0a09154b40bec3fdc3f17723df98aca7452d74c403.png'}" uri="_build/jupyter_execute/0b6a8ea915d4261d71ec8b0a09154b40bec3fdc3f17723df98aca7452d74c403.png">

/builddir/build/BUILD/myst-nb-1.4.0-build/MyST-NB-1.4.0/tests/conftest.py:316: AssertionError
----------------------------- Captured stderr call -----------------------------
[IPKernelApp] WARNING | debugpy_stream undefined, debugging will not be enabled
=========================== short test summary info ============================
FAILED tests/test_execute.py::test_complex_outputs_unrun_cache - AssertionErr...
FAILED tests/test_execute.py::test_complex_outputs_unrun_auto - AssertionErro...
FAILED tests/test_execute.py::test_custom_convert_auto - AssertionError: FILE...
FAILED tests/test_execute.py::test_custom_convert_cache - AssertionError: FIL...
FAILED tests/test_execute.py::test_custom_convert_multiple_extensions_auto - ...
FAILED tests/test_execute.py::test_custom_convert_multiple_extensions_cache

https://docs.python.org/3.15/whatsnew/3.15.html

For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.15-b1/fedora-rawhide-x86_64/10510661-myst-nb/

For all our attempts to build myst-nb with Python 3.15, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.15-b1/package/myst-nb/

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 3.15:
https://copr.fedorainfracloud.org/coprs/g/python/python3.15-b1/

Let us know here if you have any questions.

Python 3.15 is planned to be included in Fedora 45.
To make that update smoother, we're building Fedora packages with all pre-releases of Python 3.15.
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.