Block Attributes encoding and decoding on non KMIP 2.0 calls

This change adds a check to the read and write methods of the new
Attributes object that raises a new VersionNotSupported exception
if KMIP 2.0 is not the version used for encoding and decoding.
The Attributes object is not defined for older versions of KMIP
and therefore cannot be correctly encoded or decoded in those use
cases.

kmip/core/exceptions.py

@@ -324,6 +324,12 @@ class ShutdownError(Exception):
     """
 
 
+class VersionNotSupported(Exception):
+    """
+    An error generated when an unsupported KMIP version is referenced.
+    """
+
+
 class StreamNotEmptyError(Exception):
     def __init__(self, cls, extra):
         super(StreamNotEmptyError, self).__init__()