Uncommented long_description_content_type (#243)

Then Pypi also knows that the README is a markdown file so it can get rendered properly.
What confuses me is that the line has never been commented but has just been added in the commented version in a7ce75e

Author: lennartk

setup.py

@@ -54,7 +54,7 @@ def run_tests(self):
     version=version,
     description="GraphQL implementation for Python",
     long_description=open("README.md").read(),
-    # long_description_content_type="text/markdown",
+    long_description_content_type="text/markdown",
     url="https://github.com/graphql-python/graphql-core",
     download_url="https://github.com/graphql-python/graphql-core/releases",
     author="Syrus Akbary, Jake Heinz, Taeho Kim",