python-systemd fails to cross build from source for three distinct
reasons. For one thing, it Build-Depends on python3-all-dev, which
implies the host architecture interpreter and cannot be installed.
Python extensions should depend on libpython3-all-dev and
python3-all-dev:native. Once that is addressed, it runs into #1121810
and ignoring that setup.py defaults to using the build architecture
pkg-config. The build system is prepared to honour the PKG_CONFIG
environment variable, but the pybuild debhelper buildsystem does not
export it. An easy way is deferring that task to dpkg's buildtools.mk.
I'm attaching a patch for your convenience.

Helmut