After some research, this is actually not due to python-twisted but to a change in the API of python-attrs in 17.1.0 (http://www.attrs.org/en/stable/changelog.html).
But python-twisted from stretch-backports has no versioned dependency on python-attrs > 17.1.0. Installing python-attrs from stretch-backports fixes the issue. #910056 seems to concern the same problem and possibly #911771 should be merged with it. Ulrike
