pep8 -> pycodestyle

To silence this warning:

/usr/lib/python3.6/site-packages/pycodestyle.py:2190: UserWarning: [pep8] section is deprecated. Use [pycodestyle].
  warnings.warn('[pep8] section is deprecated. Use [pycodestyle].')

Change-Id: I0a2f9b266562a797d1625b3c50261e7e43315fb7
Reviewed-on: https://gerrit.libreoffice.org/61827
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

setup.cfg

-[pep8]
+[pycodestyle]
 max-line-length = 100