try without quotes

setup.cfg

@@ -10,7 +10,7 @@ author = "NYU Computation and Cognition Lab"
 author_email = "authors@psiturk.org"
 description = "An open platform for science on Amazon Mechanical Turk"
 long_description = file: README.md
-long_description_content_type = "text/markdown"
+long_description_content_type = text/markdown
 url = "https://github.com/NYUCCL/psiturk"
 
 [options]