Revert "refactor to use dict union, instead of unpacking"

This reverts commit eb6d82f.

The change itself was fine but since the code is otherwise compatible
with python 3.8, let's revert this to be compatible for one more
release.

Author: jku

tuf/api/_payload.py

@@ -346,17 +346,19 @@ def verified(self) -> bool:
     def signed(self) -> dict[str, Key]:
         """Dictionary of all signing keys that have signed, from both
         VerificationResults.
-        return a union of all signed.
+        return a union of all signed (in python<3.9 this requires
+        dict unpacking)
         """
-        return self.first.signed | self.second.signed
+        return {**self.first.signed, **self.second.signed}
 
     @property
     def unsigned(self) -> dict[str, Key]:
         """Dictionary of all signing keys that have not signed, from both
         VerificationResults.
-        return a union of all unsigned.
+        return a union of all unsigned (in python<3.9 this requires
+        dict unpacking)
         """
-        return self.first.unsigned | self.second.unsigned
+        return {**self.first.unsigned, **self.second.unsigned}
 
 
 class _DelegatorMixin(metaclass=abc.ABCMeta):