ref: fix types for integrations.mixins.issues in mypy 1.12.x (#79524)

not entirely sure why mypy wasn't able to inference this -- but this
simplified the code anyway

<!-- Describe your PR here. -->

src/sentry/integrations/mixins/issues.py

@@ -260,11 +260,7 @@ def get_defaults(self, project: Project, user: User):
 
         user_defaults = user_option_value.get(self.model.provider, {})
 
-        defaults = {}
-        defaults.update(project_defaults)
-        defaults.update(user_defaults)
-
-        return defaults
+        return {**project_defaults, **user_defaults}
 
     @abstractmethod
     def create_issue(self, data, **kwargs):