Enable flake8 for new python config files (local compilation)

Enable flake8 during local compilation of Accumulator-based python files. Will print a warning for each code violation. See atlasexternals!512 (merged) for further details. Once all packages are clear of warnings, this will be enabled for the full release build as well (see !25435 (merged)).

Packages that want to enable flake8 with the default configuration for all their python modules, can add the following to their CMakeLists.txt:

atlas_install_python_modules( python/*.py
   POST_BUILD_CMD ${ATLAS_FLAKE8} )

mentioned in merge request !25435 (merged)

This merge request affects 2 packages:

• Projects/Athena
• Projects/WorkDir

Adding @rbianchi as watcher

added Build master review-pending-level-1 labels

CI Result SUCCESS

	Athena	AthSimulation
externals
cmake
make
required tests
optional tests

Full details available at NICOS MR-25506-2019-08-08-18-56
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 1960]

@abarton I am merging this in time for the nightly.

merged

mentioned in commit 74f33012

added sweep:ignore label

mentioned in commit 4a42cd5e