Simplify isinstance call in extra_func.py (#357)

The fix in #354 used two `isinstance` calls. The second argument to
`isinstance` allows for a tuple of types, so this PR reduces the check
back down to 1 call.

Author: engnatha

protovalidate/internal/extra_func.py

@@ -332,7 +332,7 @@ def cel_is_inf(val: celtypes.Value, sign: typing.Optional[celtypes.Value] = None
 
 
 def cel_unique(val: celtypes.Value) -> celpy.Result:
-    if not isinstance(val, celtypes.ListType) and not isinstance(val, list):
+    if not isinstance(val, (celtypes.ListType, list)):
         msg = "invalid argument, expected list"
         raise celpy.CELEvalError(msg)
     return celtypes.BoolType(len(val) == len(set(val)))