flake8: cleanup error code selection

Frank Winklmeierrequested to merge
fwinkl/athena:flake8_py23 into master
  • Remove obsolete ATL238 (Py2to3 migration).
  • Move ATL902 from --enable-extensions to --select, which has the same effect (flake8 always gives priority to the most specific error code from the select/ignore lists).

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