Decrease runtime of lint job

[angela97lin]

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#2670 introduced pydocstyle and darglint packages. The darglint package specifically increases the runtime of our lint job by a few minutes. While we were okay with this addition, I suspect that there's a way to cut down on this time. Some suggestions are:

• Find a way to only lint the changed files. @freddyaboulton mentioned that scikit-learn has a way to do so via bash script that might be worth exploring: https://github.com/alteryx/evalml/pull/2670#issuecomment-918578759
• Explore github pre-commit hooks, and only call the lint job on files modified (that are not test files, and .py files).

[dsherry]

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