Rename flake8 config file to adhere to flake8 standard

[vepadulano]

As stated in https://flake8.pycqa.org/en/latest/user/configuration.html flake8 automatically find its configuration if a setup.cfg, tox.ini or .flake8 file exists in the root directory of the project. Renaming the current flake8.cfg to match the suggested naming.