Remove `pycodestyle` configuration from `setup.cfg`

Since it's now possible to move the `setuptools` configuration from `setup.cfg` into `pyproject.toml` (https://github.com/hypothesis/cookiecutters/issues/53) if we can also get the `pycodestyle` configuration out of `setup.cfg` then we could get rid of the `setup.cfg` file entirely and have one less root file.

`pycodestyle` doesn't support `pyproject.toml` and it [doesn't look like they're going to be adding support any time soon](https://github.com/PyCQA/pycodestyle/issues/813), so for the time being we have two options:

1. `pycodestyle` does support reading its configuration from `tox.ini` so we could move the `pycodestyle` config from `setup.cfg` into `tox.ini`. See https://pycodestyle.pycqa.org/en/latest/intro.html#configuration
2. We could remove `pycodestyle` from our projects. How much value does it really provide given that we have Black, isort and Pylint? There's a list of `pycodestyle`'s warnings here: https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes At a glance it doesn't look like to me like `pycodestyle` is providing any value. All of the warnings look like things that should never happen to Black- and isort-formatted code apart from a handful of warnings that I think Pylint might already cover anyway