Add py.typed file to allow users to use the types this library provides (#259)

Author: ckarnell

graphql/py.typed

@@ -82,4 +82,5 @@ def run_tests(self):
     tests_require=tests_requires,
     cmdclass={"test": PyTest},
     extras_require={"gevent": ["gevent>=1.1"], "test": tests_requires},
+    package_data={"graphql": ["py.typed"]},
 )