ci : add flake8 to github actions (python linting) (#4129)

ci : add flake8 to github actions (python linting) (#4129)

Disabled rules:

* E203 Whitespace before ':' - disabled because we often use 'C' Style where values are aligned

* E211 Whitespace before '(' (E211) - disabled because we often use 'C' Style where values are aligned

* E221 Multiple spaces before operator - disabled because we often use 'C' Style where values are aligned

* E225 Missing whitespace around operator - disabled because it's broken so often it seems like a standard

* E231 Missing whitespace after ',', ';', or ':' - disabled because we often use 'C' Style where values are aligned

* E241 Multiple spaces after ',' - disabled because we often use 'C' Style where values are aligned

* E251 Unexpected spaces around keyword / parameter equals - disabled because it's broken so often it seems like a standard

* E261 At least two spaces before inline comment - disabled because it's broken so often it seems like a standard

* E266 Too many leading '#' for block comment - sometimes used as "section" separator

* E501 Line too long - disabled because it's broken so often it seems like a standard

* E701 Multiple statements on one line (colon) - broken only in convert.py when defining abstract methods (we can use# noqa instead)

* E704 Multiple statements on one line - broken only in convert.py when defining abstract methods (we can use# noqa instead)