This was fixed upstream about 16 months ago as part of https://jelmer.uk/code/subvertpy/commit/6ff9947db9515cf7facae1db9c684fd1a0cbe14d/. There doesn't seem to have been an upstream release since then, sadly.
- Bug#942678: subvertpy ftbfs with python 3.8 (tests) Matthias Klose
- Bug#942678: Michael Hudson-Doyle
