typing: Add assertions for authentication.

Signed-off-by: Zixuan James Li <359101898@qq.com>

zproject/backends.py

@@ -1335,9 +1335,10 @@ class ExternalAuthResult:
         # more customized error messages for those unlikely races, but
         # it's likely not worth implementing.
         realm = get_realm(data["subdomain"])
-        self.user_profile = authenticate(
-            username=data["email"], realm=realm, use_dummy_backend=True
-        )
+        auth_result = authenticate(username=data["email"], realm=realm, use_dummy_backend=True)
+        if auth_result is not None:
+            assert isinstance(auth_result, UserProfile)
+        self.user_profile = auth_result
 
     class InvalidTokenError(Exception):
         pass