MCTruthClassifier: cmake cleanup, enable flake8

Remove atlas_depends_on_subdirs and simply handling of HepMC external.

This merge request affects 1 file:

• PhysicsAnalysis/MCTruthClassifier/CMakeLists.txt

Adding @zmarshal as watcher

added Analysis master review-pending-level-1 labels

CI Result SUCCESS (hash 260d4f18)

	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 23517]

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

merged

mentioned in commit 15d728d8

added sweep:ignore label