Fix numpy include path for in-tree venv

[jamestjsp]

Summary

Fix meson build error when using virtual environment inside source tree

Changes

• Convert numpy include path from absolute to relative in python/meson.build:6

Problem

Meson rejects absolute paths pointing into source tree:

ERROR: Tried to form an absolute path to a dir in the source tree.

Occurs when .venv located inside project directory.

Solution

Use os.path.relpath() to convert numpy include path to relative (matches SciPy approach from scipy/scipy#18006)

Test

• Built successfully with in-tree venv
• All 355 tests pass

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[jamestjsp]

@ilayn It was not building in virtual environment, the same issue was reported in scipy scipy/scipy#16312