DerivationFrameworkMuons: Python 3 / flake8 fixes.

Re-apply python 3 fixes clobbered by previous change. Enable flake8 checking for the python directory and fix warnings.

This merge request affects 1 package:

• PhysicsAnalysis/DerivationFramework/DerivationFrameworkMuons

added Derivation master review-pending-level-1 labels

CI Result SUCCESS (hash 7a7e65b2)

	Athena	AthSimulation	AnalysisBase	AthGeneration
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
AnalysisBase: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 13383]

merged

mentioned in commit bdc4b9da

added sweep:ignore label