chore(deps): update flake8 requirement from ^6.0 to >=6,<8 (#41)

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:production
...

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pyproject.toml

@@ -38,7 +38,7 @@ classifiers = [
 
 [tool.poetry.dependencies]
 python = "^3.8.1"
-flake8 = "^6.0"
+flake8 = ">=6,<8"
 
 [tool.poetry.group.dev.dependencies]
 astpretty = "^3.0.0"