Revert "setup: Fix long description content type to rst"

This reverts commit dbb18628.

This creates issues when uploading to pypi [1].

The type x-rst is not posing problem for example in swh.core. But in that repository the
readme is not a symlink... I'm not investigating this right now so a simple revert
should do. My focus is on deploying the opam loader on staging for now.

[1] https://jenkins.softwareheritage.org/view/swh-draft/job/DLDBASE/job/pypi-upload/111/console

setup.py

 #!/usr/bin/env python3
-# Copyright (C) 2015-2021  The Software Heritage developers
+# Copyright (C) 2015-2018  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -39,7 +39,7 @@ setup(
     name="swh.loader.core",
     description="Software Heritage Base Loader",
     long_description=long_description,
-    long_description_content_type="text/x-rst",
+    long_description_content_type="text/markdown",
     python_requires=">=3.7",
     author="Software Heritage developers",
     author_email="swh-devel@inria.fr",