Fix wheel build failure: Add scipy constraint to build-system

[jeongyoonlee]

Problem

The Ubuntu wheel build is still failing after PR #865 was merged. The build error occurs during scikit-learn installation in the build environment:

Collecting scipy>=1.10.0 (from scikit-learn>=1.6.0)
  Downloading scipy-1.17.0.tar.gz (30.4 MB)
  Run-time dependency scipy-openblas found: NO (tried pkgconfig)
  ERROR: Dependency "OpenBLAS" not found, tried pkgconfig and cmake

CI Failure: https://github.com/uber/causalml/actions/runs/21732447720/job/62689977061

Root Cause

PR #865 added scipy>=1.16.0,<1.17.0 to the runtime dependencies (under [project]), but missed adding it to [build-system] requires.

When building wheels:

1. ✅ scipy 1.16.3 installs successfully at runtime
2. ❌ But when installing build dependencies, scikit-learn>=1.6.0 requires scipy>=1.10.0
3. ❌ pip resolves this to scipy 1.17.0 (latest matching >=1.10.0)
4. ❌ scipy 1.17.0 only has manylinux_2_27 wheels, not manylinux2014
5. ❌ pip falls back to building from source → OpenBLAS missing → build fails

The Fix

Add scipy>=1.16.0,<1.17.0 to [build-system] requires:

[build-system]
requires = [
    "setuptools>=18.0",
    "wheel",
    "Cython",
    "numpy>=1.18.5",
    "scipy>=1.16.0,<1.17.0",  # ← ADDED: Pin to manylinux2014 wheels
    "scikit-learn>=1.6.0",
]

This ensures scipy 1.16.3 (which has manylinux2014 wheels) is installed before scikit-learn in the build environment, preventing the source build attempt.

Why This Wasn't in PR #865

PR #865 was merged before this issue was fully diagnosed. The initial fix only addressed runtime dependencies, and the build-system issue was discovered later when analyzing the continued CI failures.

Testing

Once merged, the "Upload Python Package" workflow should successfully build wheels without OpenBLAS errors.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes Ubuntu wheel build failures by adding the scipy version constraint to the [build-system] requires section. This follows up on PR #865, which only added the constraint to runtime dependencies, missing that the build system also needs this constraint to prevent pip from selecting scipy 1.17.0 during the build process.

Changes:

• Added scipy>=1.16.0,<1.17.0 to [build-system] requires to ensure manylinux2014-compatible wheels are used during the build process

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