Make UserType an attrs class

federation/types.py

@@ -1,6 +1,9 @@
 from typing import Optional
 
+import attr
 
+
+@attr.s
 class UserType:
-    id: str
+    id: str = attr.ib()
-    private_key: Optional[str]
+    private_key: Optional[str] = attr.ib(default=None)

setup.py

@@ -27,6 +27,7 @@ setup(
     packages=find_packages(),
     license="BSD 3-clause",
     install_requires=[
+        "attrs",
         "cssselect>=0.9.2",
         "dirty-validators>=0.3.0",
         "lxml>=3.4.0",