Fix #550 Pin flake8 to < 6 to allow pip-compile to succeed

[gopackgo90]

flake8 6 drops support for python 3.7, prospector still supports python 3.7.

Description

Flake8 6 drops python 3.7 which prospector still supports, pinning flake8 to less than 6 should allow pip-compile to work on pythons 3.8+

Related Issue

#550

Motivation and Context

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change to the dependencies
• I have updated the dependencies accordingly
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Pierre-Sassoulas]

Thanks, I agree with your analysis.