define -> frozen

src/allmydata/grid_manager.py

@@ -25,11 +25,12 @@ from allmydata.util import (
 
 from attrs import (
     define,
+    frozen,
     Factory,
 )
 
 
-@define
+@frozen
 class SignedCertificate(object):
     """
     A signed certificate.

src/allmydata/test/test_grid_manager.py

@@ -434,11 +434,14 @@ class GridManagerInvalidVerifier(SyncTestCase):
         An incorrect signature is rejected
         """
         # make signature invalid
-        self.cert0.signature = invalid_signature.encode("ascii")
+        invalid_cert = SignedCertificate(
+            self.cert0.certificate,
+            invalid_signature.encode("ascii"),
+        )
 
         verify = create_grid_manager_verifier(
             [self.gm._public_key],
-            [self.cert0],
+            [invalid_cert],
             ed25519.string_from_verifying_key(self.pub0),
             bad_cert = lambda key, cert: None,
         )