Add long_description_content_type to package metadata

setup.py

@@ -172,6 +172,7 @@ setup(
     author_email='georg@python.org',
     description='Python documentation generator',
     long_description=long_desc,
+    long_description_content_type='text/x-rst',
     zip_safe=False,
     classifiers=[
         'Development Status :: 5 - Production/Stable',