Update isort requirement from ==5.7.* to ==5.8.*

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/develop/CHANGELOG.md)
- [Commits](PyCQA/isort@5.7.0...5.8.0)

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

Author: dependabot[bot]

requirements-test.txt

@@ -2,6 +2,6 @@ pytest==6.2.*
 pytest-cov==2.11.*
 pytest-vcr==1.0.*
 black==20.8b1
-isort==5.7.*
+isort==5.8.*
 flake8==3.8.*
 mypy==0.812