change: enforce some docstyle conventions

[metrizable]

Issue #, if available:

Description of changes:

adding docstyle target invoking pydocstyle

Testing done:

unit:

❯ tox --parallel 3 -- -rfE --disable-warnings tests/unit
✔ OK black-format in 10.858 seconds
✔ OK flake8 in 24.66 seconds
✔ OK pylint in 34.98 seconds
✔ OK docstyle in 24.452 seconds
✔ OK twine in 8.569 seconds
✔ OK doc8 in 15.286 seconds
✔ OK sphinx in 36.146 seconds
✔ OK py36 in 4 minutes, 36.81 seconds
✔ OK py37 in 5 minutes, 5.786 seconds
✔ OK py38 in 4 minutes, 55.6 seconds

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AWS CodeBuild CI Report

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• Commit ID: 652e6c9
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AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: 652e6c9
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AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 652e6c9
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AWS CodeBuild CI Report

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• Commit ID: 652e6c9
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