build(deps-dev): bump isort from 5.8.0 to 5.9.1

Bumps [isort](https://github.com/pycqa/isort) from 5.8.0 to 5.9.1.
- [Release notes](https://github.com/pycqa/isort/releases)
- [Changelog](https://github.com/PyCQA/isort/blob/main/CHANGELOG.md)
- [Commits](https://github.com/pycqa/isort/compare/5.8.0...5.9.1)

---
updated-dependencies:
- dependency-name: isort
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

setup.py

@@ -72,7 +72,7 @@ setuptools.setup(
             "pytest-cov==2.12.1",
             "pycodestyle==2.7.0",
             "black==21.6b0",
-            "isort==5.8.0",
+            "isort==5.9.1",
             "mypy==0.901",
             # type stubs for mypy
             "types-backports==0.1.2",