Bump isort from 4.3.21 to 5.9.3

Bumps [isort](https://github.com/pycqa/isort) from 4.3.21 to 5.9.3.
- [Release notes](https://github.com/pycqa/isort/releases)
- [Changelog](https://github.com/PyCQA/isort/blob/main/CHANGELOG.md)
- [Commits](PyCQA/isort@4.3.21...5.9.3)

---
updated-dependencies:
- dependency-name: isort
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

requirements/testing.txt

@@ -1,5 +1,5 @@
 -r production.txt
-isort==4.3.21
+isort==5.9.3
 coveralls==2.0.0
 flake8==3.8.1
 flake8-isort==2.8.0