Pin Sphinx version.

See breathe-doc/breathe#981

docker/Dockerfile.test-env

@@ -154,7 +154,7 @@ RUN if [ "$MPI" = "mpich" ]; then \
 #   documentation or run tests.
 RUN pip3 install --no-cache-dir --upgrade setuptools pip && \
     pip3 install --no-cache-dir cffi mpi4py numba numpy==${NUMPY_VERSION} scikit-build-core[pyproject] && \
-    pip3 install --no-cache-dir breathe clang-format cmakelang jupytext matplotlib mypy myst-parser nanobind==${NANOBIND_VERSION} pytest pytest-xdist ruff scipy sphinx sphinx_rtd_theme types-setuptools
+    pip3 install --no-cache-dir breathe clang-format cmakelang jupytext matplotlib mypy myst-parser nanobind==${NANOBIND_VERSION} pytest pytest-xdist ruff scipy sphinx==7.2.6 sphinx_rtd_theme types-setuptools
 
 # Install KaHIP
 RUN wget -nc --quiet https://github.com/kahip/kahip/archive/v${KAHIP_VERSION}.tar.gz && \