`packaging_test` fails: `long_description` has syntax errors in markup and would not be rendered on PyPI.

[hf-kklein]

ERROR long_description has syntax errors in markup and would not be
rendered on PyPI.
line 99: Warning: Duplicate explicit target name: "contribution guide".
Checking dist/bo4e-0.1.dev1+gcdc11af.tar.gz: FAILED
ERROR long_description has syntax errors in markup and would not be
rendered on PyPI.
line 99: Warning: Duplicate explicit target name: "contribution guide".

[hf-kklein]

Fixed in #873