Flake8-isort

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- Support flake8 4.x [g-as]

- Switch from travis-ci to github actions. [g-as]

- Drop python 2.7 support and 3.5 as well [g-as]

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- Nothing changed yet.

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- support isort >= 5 [bnavigator, pkolbus]

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- Work around `FailedToLoadPlugin` exception by requiring `isort` 4.x. Likewise,
pin the major version of all dependencies, to reduce risk of any future
incompatibilities.
[pkolbus]

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- Let `isort` search the configuration, rather than `flake8-isort` try to find it.
[jnns]

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- Fix flake8 warning.
[sobolevn]