Fix long description of Python package

[leplatrem]

I was using the longtest command from zest.releaser in order to find issues in our package description.

Ideally we could add a test in CI (opened for that purpose https://github.com/zestsoftware/zest.releaser/pull/380)

[leplatrem]

bors merge

[bors[bot]]

Build succeeded:

• ci/circleci: build_test_publish