pycryptodome fails to cross build from source for multiple reasons most
of which relate to its Build-Depends.

The sphinx-related packages are not cross-satisfiable. Fortunately, the
documentation has already been separated to a -doc package so it should
be possible to move the dependencies to B-D-I. For doing so, the build
steps (in execute_after_dh_auto_build) and the debhelper addon must be
skipped in arch-only builds. Beyond thus, the python3-all-dev dependency
needs to be multiarchified. I'm attaching a patch with the necessary
changes. Once applying it, pycryptodome could be cross built if it was
not running into #1121810.

Helmut