Add py.typed (pep-0561). Fix #8 (#9)

Author: penguinolog

advanced_descriptors/py.typed

@@ -256,6 +256,8 @@ def get_simple_vars_from_src(src):
         'advanced_descriptors': [
             os.path.basename(filename)
             for filename in glob.glob(os.path.join('advanced_descriptors', '*.pyi'))
+        ] + [
+            'py.typed'
         ],
     },
 )