fix: Specify mlio version 0.2.7

[wiltonwu]

Issue #, if available: mlio was recently upgraded to 0.3.X which introduced breaking API changes. See https://github.com/aws/sagemaker-scikit-learn-extension/issues/9

Description of changes: This PR specifies the dependency of mlio to 0.2.7 (latest stable version of 0.2.X).

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[wiltonwu]

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• CodeBuild project: sagemaker-sklearn-extension-pr
• Commit ID: 284641363ce00647b6d5f93f2f909ad17ab58dc9
• Result: SUCCEEDED
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