Revert long_description_content_type to 'text/x-rst'

9474ab23 was a workaround for this role being
unknown to PyPI.

Removing the role fixes the issue. I also do not think it is very useful;
all loaders have this entry point.

docs/README.rst

@@ -4,11 +4,12 @@ Software Heritage - CVS loader
 The Software Heritage CVS Loader imports the history of CVS repositories
 into the SWH dataset.
 
-The main entry points are
+The main entry points is:
 
--  :class:`swh.loader.cvs.loader.CvsLoader` for the main cvs loader
+-  ``swh.loader.cvs.loader.CvsLoader`` for the main cvs loader
    which ingests content out of a local cvs repository
 
+
 Features
 --------
 

setup.py

@@ -40,7 +40,7 @@ setup(
     name="swh.loader.cvs",
     description="Software Heritage CVS Loader",
     long_description=long_description,
-    long_description_content_type="text/markdown",
+    long_description_content_type="text/x-rst",
     python_requires=">=3.7",
     author="Software Heritage developers",
     author_email="swh-devel@inria.fr",