Add Pixi lockfiles to ensure Python environment reproducibility

[marcelovilla]

Reference Issues or PRs

Closes #204
Closes #173 (indirectly)

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Documentation

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• This content adheres to the Nebari style guides.

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Any other comments?

[marcelovilla]

We were not adding Python as an explicit dependency before, but I suggest we do. I'm pinning it to the major version found in quay.io/nebari/nebari-jupyterlab:main-e1418d0-20250304

[marcelovilla]

We were not adding Python as an explicit dependency before, but I suggest we do. I'm pinning it to the major version found in quay.io/nebari/nebari-jupyterhub:main-e1418d0-20250304

[marcelovilla]

Commented out until #209 is addressed