Although the tests are passing locally, it seems as the excessive
filesystem usage causes several build failures and timeouts in the Hydra
and OfBorg infrastructure:
* https://hydra.nixos.org/build/84374861 (python3 on linux.x86_64)
* https://hydra.nixos.org/build/84368459 (python2 on linux.x86_64)
Some of these tests are failing after several seconds though, but I
couldn't identify a pattern and I'm not overly surprised that a FTP
library has impure tests. However the API seems to be usable in a Python
{2,3} environment, so it should be safe to use even with disabled tests.
