Update flake8 requirement from <6,>=4 to >=4,<7

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@4.0.0...6.0.0)

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

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

Author: dependabot[bot]

setup.py

@@ -68,7 +68,7 @@
             "bandit>=1.6,<2",  # Checks code for security issues
             "coverage>=5.3,<7",  # Lets us track what code is being tested
             "doc8>=0.9,<1.1",  # Ensures clean documentation formatting
-            "flake8>=4,<6",  # A framework for linting & static analysis
+            "flake8>=4,<7",  # A framework for linting & static analysis
             "flake8-builtins>=1.5,<3",  # Avoid shadowing Python built-in names
             "flake8-colors>=0.1,<0.2",  # Produce colorful error / warning output
             "flake8-docstrings>=1.5,<2",  # Ensure docstrings are formatted well