dulwich: backporting fix for bytes problem in alternates
Sub-classing Dulwich whole testcase brings in all the tests from that class, but we don't care: they pass in less than a second.
Given the release rate of dulwich, a new version should be available soon, but we haven't tested anything from 0.20.5. It doesn't look like the best time to update. In any case the backport gives us more options.
Closes heptapod#370 (closed)