Revert "pin flake8 at 3.7.9 (#679)" (#1017)
Revert "pin flake8 at 3.7.9 (#679)" (#1017)

This reverts commit 021723a5c1cdde8cd15b542179e1fd83bd32819b.
Underlying issue was fixed by commit 20cf4cb087371.
open-telemetry/opentelemetry-python,open-telemetry/opentelemetry-python