GH-48172: [Python] Add cp315 to build

[paultiq]

Rationale for this change

Add Python 3.15 to the pyarrow nightly builds, so that issues can be detected early in the development cycles of both cpython & pyarrow.

Note:

• CFFI and Pandas don't have nightly 3.15 wheels, and are accordingly excluded.
• Numpy doesn't ship a 3.15 wheel, but is needed to build. It is building from source.

What changes are included in this PR?

• Adding cp315/cp315t to the build matrix
• Excluding cffi/pandas for 3.15, as the wheels aren't yet available.

Are these changes tested?

• Yes, tested on a local Linux build using the ninja-release-python preset (not maximal)

Are there any user-facing changes?

No functional changes, other than allowing the user to use Python 3.15.

• GitHub Issue: [Python] Add support and wheels for Python 3.15 #48172

[github-actions]

⚠️ GitHub issue #48172 has been automatically assigned in GitHub to PR creator.

[raulcd]

@github-actions crossbow submit wheel-manylinux-2-28-cp315-cp315-amd64

[github-actions]

Revision: 2d2a557

Submitted crossbow builds: ursacomputing/crossbow @ actions-c8d038a747

Task	Status
wheel-manylinux-2-28-cp315-cp315-amd64

[paultiq]

The build was destined for failure, because cp315 is not yet in the manylinux build. I'll reproduce locally and see if there's a graceful way to resolve, along with a clearer failure when PYTHON_ROOT is not found.

The build failed oddly, but the earlier issue was that the detection of PYTHON_ROOT failed quietly:

2025-11-21T08:07:23.9363828Z #20 [stage-0 13/16] RUN PYTHON_ROOT=$(find /opt/python -name cp315-cp315) && echo "export PATH=$PYTHON_ROOT/bin:$PATH" >> /etc/profile.d/python.sh
...
2025-11-21T08:08:01.3779653Z -- Found Python3: /bin/python3.11 (found version "3.11.11") found components: Interpreter

A test should be added to verify PYTHON_ROOT is set / not empty and fail fast:

arrow/ci/docker/python-wheel-manylinux.dockerfile

Lines 110 to 111 in 2fb2f79

	RUN PYTHON_ROOT=$(find /opt/python -name cp${PYTHON_VERSION/./}-${PYTHON_ABI_TAG}) && \
	echo "export PATH=$PYTHON_ROOT/bin:\$PATH" >> /etc/profile.d/python.sh

Build Failure

Crossbow failed due to

lto1: fatal error: bytecode stream in file ‘src/arrow/CMakeFiles/arrow_vendored.dir/Unity/unity_1_c.c.o’ generated with GCC compiler older than 10.0

caused by

2025-11-21T08:08:00.9596950Z -- The C compiler identification is GNU 8.5.0
. 2025-11-21T08:08:01.0402969Z -- The CXX compiler identification is GNU 14.2.1

[paultiq]

Looking more closely, and considering the history of 3.13 and 3.14:

This (nightlys) needs to wait manylinux adds cp315. The alternative is to add a cpython build step or use one of the other pre-built images, but this seems like more work than other projects are investing at this stage.

Based on history of 3.13/3.14, this would be after b1 / 2026-05-05.