Fix linting issues (#1412)

Author: firaskafri

setup.py

@@ -38,7 +38,7 @@
     version=version,
     description="Graphene Django integration",
     long_description=open("README.md").read(),
-    long_description_content_type='text/markdown',
+    long_description_content_type="text/markdown",
     url="https://github.com/graphql-python/graphene-django",
     author="Syrus Akbary",
     author_email="me@syrusakbary.com",