manually specify multiprocessing use fork in setup.py

[IanDelbridge]

Proposed changes

This fixes a bug (cython/cython#3262) related to compiling Cython with nthreads>0 in python >=3.8 on MacOS.

Alternatives:

• always setting nthreads to 0
• explicitly checking for MacOS and python>=3.8

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[jeongyoonlee]

I can't reproduce the issue with master, Cython=0.29.34, Python3.8, and macOS (M1, Sonoma). What environment are you using?

[IanDelbridge]

Oh interesting. I apologize - I should have included more about my environment. I am using

• Cython 0.29.34
• python 3.8.17
• causalml 0.15.1.dev0 built from master, see below
• macOS Sonoma Version 14.3 (M2 Max)

I built this environment with the following steps:

pyenv virtualenv 3.8.17 fresh-causalml-developer-py38
pyenv shell fresh-causalml-developer-py38
cd causalml/
git checkout master
git pull origin
pip install ".[test]"
make build // results in the cython issue linked above. 

[jeongyoonlee]

Thanks @IanDelbridge. I was able to reproduce both the issue and the resolution.

[jeongyoonlee]

LGTM. Thanks!