Avoid newer isort for flake8 tests
Avoid newer isort for flake8 tests

Do not install the latest isort because it currently breaks
flake8-isort. This pinning should be removed as soon as
https://github.com/gforcada/flake8-isort/issues/88 is resolved.

arthurdejong/python-stdnum,arthurdejong/python-stdnum,arthurdejong/python-stdnum