Update isort requirement from <=5.6.4,>=5.5.2 to >=5.5.2,<5.7.1

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@5.5.2...5.7.0)

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

requirements.txt

@@ -4,7 +4,7 @@ attrs>=20.3.0
 click>=7.1.2
 consolekit>=0.9.0
 domdf-python-tools>=2.4.0
-isort<=5.6.4,>=5.5.2
+isort>=5.5.2,<5.7.1
 toml>=0.10.2
 typing-extensions>=3.7.4.3
 yapf==0.30.0