Bug#1115771: Missing pytest-asdf-plugin

[Ole Streicher]

Control: block -1 by 1115708

This FTBFS is caused by an updated package python3-asdf, which with 
version 5.0.0 no longer includes the pytest_asdf Python package. 
pytest_asdf is now a separate package, but still waiting in NEW:

https://ftp-master.debian.org/new/pytest-asdf-plugin_0.1.1-1.html

After the package is accepted, python3-pytest-asdf-plugin must be added 
as test dependency.