Mark the package as containing type hints

See PEP 561: https://www.python.org/dev/peps/pep-0561/

Author: David Robertson

setup.py

@@ -49,4 +49,7 @@ def read_file(path_segments):
     },
     long_description=read_file(("README.rst",)),
     keywords="json",
+    package_data={
+        "signedjson": ["py.typed"]
+    }
 )