`pep8` is nowadays called `pycodestyle`

See <https://github.com/PyCQA/pycodestyle/issues/466>. Pointing towards
the `pep8` tool risks leading users to outdated versions of the tool
(the last `pep8` version is 1.7.0 from January 2016; the currently
latest `pycodestyle` version is 2.3.0).