infra: fix tox test dependencies and bump coverage threshold to 86%

[laurenyu]

Issue #, if available:

Description of changes:
as I discovered in #1633, Python 3.8 introduced a change that required changes in coverage. This has been fixed in coverage>=4.5.2, and even though the builds were claiming that coverage==5.1 was installed, the coverage check was failing for only Python 3.8. Turns out tox has some interesting caching behavior.

Testing done:
tox -e py38 tests/unit (and tried it out in #1633)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: 133a34e
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-pr
• Commit ID: 133a34e
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 133a34e
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 133a34e
• Result: SUCCEEDED
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[metrizable]

ship it!