DerivationFrameworkTau: Python 3 / flake 8 fixes

Updates for python 3 compatibility.

Also enable flake8 checking and fix warnings.

This merge request affects 1 package:

• PhysicsAnalysis/DerivationFramework/DerivationFrameworkTau

added Derivation Tau master review-pending-level-1 labels

CI Result SUCCESS (hash c27baef0)

	Athena	AthSimulation
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
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 10357]

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

merged

mentioned in commit 0b183604

added sweep:ignore label