Bump flake8-isort from 2.9.0 to 2.9.1 (#1301)

Bumps [flake8-isort](https://github.com/gforcada/flake8-isort) from 2.9.0 to 2.9.1.
- [Release notes](https://github.com/gforcada/flake8-isort/releases)
- [Changelog](https://github.com/gforcada/flake8-isort/blob/master/CHANGES.rst)
- [Commits](gforcada/flake8-isort@2.9.0...2.9.1)

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