Tidy ISF HepMC tools new config, add flake8 test

Remove incorrect "to be migrated" comments.

Add post-build flake8 test, and make minor tweaks to old config to pass without warnings.

(new and old config kept in separate commits)

added NewConfig label

changed the description

This merge request affects 1 package:

• Simulation/ISF/ISF_HepMC/ISF_HepMC_Tools

Adding @jchapman ,@vpascuzz as watchers

added Simulation master review-pending-level-1 labels

CI Result SUCCESS (hash bcda5a39)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 18130]

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit c17dc2bd

added sweep:ignore label