Update flake8-isort requirement from <5,>=4 to >=4,<7 in /requirements

Updates the requirements on [flake8-isort](https://github.com/gforcada/flake8-isort) to permit the latest version.
- [Release notes](https://github.com/gforcada/flake8-isort/releases)
- [Changelog](https://github.com/gforcada/flake8-isort/blob/master/CHANGES.rst)
- [Commits](gforcada/flake8-isort@4.0.0...6.0.0)

---
updated-dependencies:
- dependency-name: flake8-isort
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/development.txt

@@ -3,6 +3,6 @@
 
 black
 flake8-builtins>=2,<3
-flake8-isort>=4,<5
+flake8-isort>=4,<7
 flake8-qgis>=1,<2
 pre-commit>=3,<4