Add pyproject.toml with basic build dependencies for PEP518 compliance

[darthtrevino]

Proposed changes

I ran into errors when installing this library using Poetry, but by adding a pyproject.toml file with the dependencies necessary at install/build time I was able to resolve this.

Fixes #551

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)

Checklist

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• I have read the CONTRIBUTING doc
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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 (see dowhy PR pointing at my fork)

[darthtrevino]

@jeongyoonlee, could you review this one? I'm doing some maintenance updates on dowhy, and this is blocking us from upgrading to Poetry 1.2

[jeongyoonlee]

LGTM. Thanks for the contribution!

[heiderich]

@jeongyoonlee Do you plan to release a new version of causalml any time soon? With this PR merged, this would hopefully make poetry installations easier.

[jeongyoonlee]

Apologize for the delay. The CausalML team has been busy with the upcoming Causal ML KDD workshop. We plan to release a new version by the end of the month. Thanks.