typing: Simplify return type of RequestContext.to_policy_values

The specific type used here is irrelevant.

Change-Id: I9569a3769e901c75f05bbd272260620be5ed382c
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

oslo_context/context.py

@@ -238,7 +238,7 @@ class RequestContext:
         """Store the context in the current thread."""
         _request_store.context = self
 
-    def to_policy_values(self) -> _DeprecatedPolicyValues:
+    def to_policy_values(self) -> _MutableMapping:
         """A dictionary of context attributes to enforce policy with.
 
         oslo.policy enforcement requires a dictionary of attributes