oggvideotools and 300 other source packages cannot satisfy their cross
Build-Depends, because their transitive dependency on python3-defusedxml
is not satisfiable. In general, Architecture: all packages can never
satisfy cross Build-Depends unless marked Multi-Arch: foreign or
annotated :native. Since python3-defusedxml is a pure Python module with
no architecture-dependent dependencies of its own, it is sensible to
attach the foreign marking. Please consider applying the attached patch.

Helmut