Remove packaging for Python 3.6, 3.7, and 3.9

[jeongyoonlee]

Proposed changes

Currently the automated packaging workflow fails because it builds and uploads the same artifact across different Python versions, and PyPI doesn't allow to upload a file that already exists (e.g. v0.12.1 packaging workflow).

This PR will remove packaging for Python 3.6, 3.7 and 3.9, and leave only one packaging workflow for Python 3.8.

Types of changes

What types of changes does your code introduce to CausalML?
Put an x in the boxes that apply

• 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

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING doc
• I have signed the CLA
• 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

N/A.

[paullo0106]

LGTM!