Update isort requirement from ==5.8.* to ==5.9.*

Updates the requirements on [isort](https://github.com/pycqa/isort) to permit the latest version.
- [Release notes](https://github.com/pycqa/isort/releases)
- [Changelog](https://github.com/PyCQA/isort/blob/main/CHANGELOG.md)
- [Commits](PyCQA/isort@5.8.0...5.9.3)

---
updated-dependencies:
- dependency-name: isort
  dependency-type: direct:production
...

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

requirements-test.txt

@@ -1,8 +1,8 @@
 black==21.5b1
 flake8==3.8.*
-isort==5.8.*
+isort==5.9.*
 mock==4.0.3
-mongomock==3.22.*
+mongomock==3.23.*
 mypy==0.812
 pytest==6.2.*
 pytest-cov==2.12.*