Update flake8 requirement from ^6.0.0 to >=6,<8 (#11)

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Commits](PyCQA/flake8@6.0.0...7.0.0)

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

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pyproject.toml

@@ -17,7 +17,7 @@ psycopg = "^3.1.8"
 pytest-mock = "^3.10.0"
 
 [tool.poetry.group.dev.dependencies]
-flake8 = "^6.0.0"
+flake8 = ">=6,<8"
 black = "^23.1.0"
 mypy = "^1.0.0"
 psycopg-binary = "^3.1.8"