[Debian-med-packaging] Bug#972812: python-cobra: FTBFS with Python 3.9 as supported

[Graham Inggs]

Control: affects -1 src:libsbml

The problem here is libsbml only builds its python extension for the
default python, while python-cobra is trying to test all supported
python versions.

Either libsbml should be fixed to build extensions for all supported
versions, or python-cobra should be fixed to test only the default
python.

Thanks Michael Hudson-Doyle for this hint.