So it seems that this is indeed a runtime incompatibility - which makes sense with mitmproxy binary having a versioned dependency on python3-tornado (>= 4.3) right now. I think the best way forward would be to update mitmproxy to 5.1.1, if we don't have enough confidence that the outlined commit is enough of course.
Cheers, -- Ćukasz 'sil2100' Zemczak Foundations Team lukasz.zemc...@canonical.com www.canonical.com