Use dashes for pycodestyle max line length config (#3513)

The option is `max-line-length` with dashes, not underscores. The config option name is given in the output of `pycodestyle -h`, which can also be checked on https://pep8.readthedocs.io/en/stable/intro.html#example-usage-and-output:
```
Configuration:
    The project options are read from the [pycodestyle] section of the
    tox.ini file or the setup.cfg file located in any parent folder of the
    path(s) being processed.  Allowed options are: exclude, filename,
    select, ignore, max-line-length, max-doc-length, hang-closing, count,
    format, quiet, show-pep8, show-source, statistics, verbose
```