Apply isort to python_package and corresponding tests

[zyxue]

Summary

Apply isort to all python files, including tests

Motivation

Consistent imports order improves readability

Description

Per discussion at #3957 (comment), isort will be applied to the following files, which are selected with

fd --exclude external_libs -F '.py' | sort

one PR at a time for ease of review:

• docs/conf.py
• examples/python-guide/advanced_example.py
• examples/python-guide/logistic_regression.py
• examples/python-guide/plot_example.py
• examples/python-guide/simple_example.py
• examples/python-guide/sklearn_example.py
• helpers/check_dynamic_dependencies.py
• helpers/parameter_generator.py
• python-package/lightgbm/__init__.py
• python-package/lightgbm/basic.py
• python-package/lightgbm/callback.py
• python-package/lightgbm/compat.py
• python-package/lightgbm/dask.py
• python-package/lightgbm/engine.py
• python-package/lightgbm/libpath.py
• python-package/lightgbm/plotting.py
• python-package/lightgbm/sklearn.py
• python-package/setup.py
• tests/c_api_test/test_.py
• tests/cpp_test/test.py
• tests/python_package_test/__init__.py
• tests/python_package_test/test_basic.py
• tests/python_package_test/test_consistency.py
• tests/python_package_test/test_dask.py
• tests/python_package_test/test_dual.py
• tests/python_package_test/test_engine.py
• tests/python_package_test/test_plotting.py
• tests/python_package_test/test_sklearn.py
• tests/python_package_test/test_utilities.py
• tests/python_package_test/utils.py

References

[StrikerRUS]

@zyxue Thank you very much for your great contribution and original feature request!

[github-actions]

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