CI: Use the latest pep8speaks and remove custom configuration
Until now, pep8speaks used independent settings, and not the setuptools standard `setup.cfg`. This has been addressed in the latest version of pep8speaks, and we shouldn't need to repeat our configuration anymore: https://github.com/pandas-dev/pandas/blob/master/.pep8speaks.yml#L10

More info about the problem can be found here: https://github.com/OrkoHunter/pep8speaks/issues/95#issuecomment-470887715

Not sure how pep8speaks is integrated, but we should upgrade to the latest version if it's not automatic, and we should remove the duplicated configuration in `.pep8speaks.yml` (which is already present in `setup.cfg`).

