pam-python cannot be cross built from source, because its sphinx
dependency is not satisfiable. Fortunately, the documentation is
separated to an arch:all package. Therefore sphinx can be skipped in
arch-only builds such as cross builds. I'm attaching a patch that fully
separates the library and the documentation build into arch/indep. I
used reproducible builds to verify that this refactoring does not change
output artifacts. I note that there was a --system=pybuild flag that was
ignored by debhelper to select a build system, --buildsystem should have
been used. I dropped the confusing flag and retained the detected
makefile buildsystem.

Helmut