Add flake8 linter for Python files in CI/CD

[sandipanpanda]

What this PR does / why we need it:
Check the style and quality of python code using flake8 in CI/CD

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes # #1910

[google-oss-prow]

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[andreyvelich]

Thank you for your contribution @sandipanpanda!

I think the main idea behind this issue #1910 was to add another instruction to the Makefile which will run script from hack directory (e.g. verify-python.sh).
This script will use black and flake8 to check Python code.

So contributors can easily run make fmt-python.
Does it make sense ?