AthAnalysisExternals: add flake8 packages

Add PyModules and flake8_atlas to the list of packages. We can then configure flake8 checking for the AthAnalysis project as well. Might be useful for those developers that develop code within that project to avoid warnings only showing up later in the CI.

Ideally, I would have liked adding it to AnalysisBase as well but that's a bit more complicated as we cannot rely on pip (taken from LCG) for installation.

cc @krumnack

Let me add @akraszna who handles our externals.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST-CC7 #475]

mentioned in commit f299fcc6

merged