setup.py: Fix description content-type to markdown
Otherwise, the upload-to-pypi build complains about it [1] [1] https://jenkins.softwareheritage.org/job/DLDCVS/job/pypi-upload/12/console
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Otherwise, the upload-to-pypi build complains about it [1] [1] https://jenkins.softwareheritage.org/job/DLDCVS/job/pypi-upload/12/console