fix: libprotobuf==3.11.4 is not backwards compatible, specify tox version for testing

[wiltonwu]

Description of changes:

• mlio is broken because libprotobuf released 3.11.4 which is not compatible with other versions of 3.11.X (consumed by mlio)
• This release caused our build systems, sanity checks, and local runs of tox to fail
• This change fixes the version of libprotobuf to 3.11.0 (confirmed with @cbalioglu to work)

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[wiltonwu]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-sklearn-extension-pr
• Commit ID: bd755daa05c22482d9c8d4502b3dfc3367466635
• Result: SUCCEEDED
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