setup.py: pin pycodestyle for flake8 3.6.0
setup.py: pin pycodestyle for flake8 3.6.0

Somehow again, we're not resolving the dependency for flake8 correctly
automatically.

Both autopep8 and flake8 depend on pycodestyle.

    flake8 3.6.0 has requirement pycodestyle<2.5.0,>=2.4.0, but you have
    pycodestyle 2.3.1.

    autopep8 1.4.3 has requirement pycodestyle>=2.4.0, but you have
    pycodestyle 2.3.1.

Pin pycodestyle to the 2.4 series, which will keep both happy for now.

aevri/mel,aevri/mel