Bump isort from 5.2.0 to 5.6.4 in /src/requirements (#674)

Bumps [isort](https://github.com/pycqa/isort) from 5.2.0 to 5.6.4.
- [Release notes](https://github.com/pycqa/isort/releases)
- [Changelog](https://github.com/PyCQA/isort/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/pycqa/isort/compare/5.2.0...5.6.4)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

src/requirements/dev.txt

@@ -6,6 +6,6 @@ coverage==5.2.1
 codecov==2.1.8
 black==19.10b0
 pre-commit==2.8.1
-isort==5.2.0
+isort==5.6.4
 hypothesis==5.39.0
 flake8==3.8.3