Fix content type (#1122)

setup.cfg

@@ -4,7 +4,7 @@ author = Quansight
 url = https://github.com/quansight/qhub
 description = Management of QHub on Cloud Infrastructure
 long_description = file: README.md
-long_description_type = text/markdown
+long_description_content_type = text/markdown
 keywords = aws, gcp, do, azure, qhub
 license = BSD 3-Clause License
 classifiers =