Update flake8 version to fix compatibility error.

[tpatki]

Description

Update flake8 version.
Fixes #580

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature/architecture support (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Build/CI update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please provide hardware architecture specs and
instructions so we can reproduce.

• Test A: Hardware architecture, machine name, example/test run
• Test B: Hardware architecture, machine name, example/test run
• ...

Checklist:

• I have run ./scripts/check-code-format.sh and confirm my code code follows the style guidelines of variorum
• I have added comments in my code
• My changes generate no new warnings (build with -DENABLE_WARNINGS=ON)
• New and existing unit tests pass with my changes

Thank you for taking the time to contribute to Variorum!