tests: disable datagram bytes() test on Python 2

See #297

tests/putil/test_datagram.py

@@ -1,5 +1,6 @@
 import pytest
 from panda3d import core
+import sys
 
 # Fixtures for generating interesting datagrams (and verification functions) on
 # the fly...
@@ -76,6 +77,7 @@ def datagram_large():
 
     return dg, readback_function
 
[email protected](sys.version_info < (3, 0), reason="Requires Python 3")
 def test_datagram_bytes():
     """Tests that we can put and get a bytes object on Datagram."""
     dg = core.Datagram(b'abc\x00')