Opens up scipy dependency version range towards newer releases (#441)

[tonkolviktor]

Proposed changes

Package requires old scipy version (#441)

Types of changes

What types of changes does your code introduce to CausalML?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

I did the following

pytest -vs tests/ --cov causalml/   # test if all works at start
python -m pip install scipy==1.8.0  # install latest scipy
pytest -vs tests/ --cov causalml/   # run tests again to check if all still works

I'm unsure if anything else is required.

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[tonkolviktor]

I've signed this cla

[jeongyoonlee]

LGTM. To be thorough, it'd good to check if example notebooks run fine. With this PR, however, I don't expect any changes in example notebooks. Thanks!