Event/xAOD/xAOD*Cnv: enable flake8 and make code compliant

Frank Winklmeierrequested to merge
fwinkl/athena:fwinkl_20201029T141653 into master

Enable flake8 for Event/xAOD/xAOD*Cnv packages:

  • remove import *
  • remove unnecessary code for logger restoration
  • use is None instead of == None, etc.

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