future first: switch the order of some if statements (#5206)

Since we're adding this to our contribution guidelines in
https://github.com/python/typeshed/pull/5205

stubs/protobuf/google/protobuf/message.pyi

@@ -10,10 +10,10 @@ class EncodeError(Error): ...
 
 _M = TypeVar("_M", bound=Message)  # message type (of self)
 
-if sys.version_info < (3,):
-    _Serialized = Union[bytes, buffer, unicode]
-else:
+if sys.version_info >= (3, 0):
     _Serialized = ByteString
+else:
+    _Serialized = Union[bytes, buffer, unicode]
 
 class Message:
     DESCRIPTOR: Descriptor