Throw an exception on invalid BufferView instantiation

Also adjust tests to skip mmap under Python 2.

test/test_serialization.py

@@ -6,6 +6,7 @@ import test_regression
 import tempfile
 import pickle
 import mmap
+import sys
 
 this_dir = os.path.dirname(__file__)
 
@@ -41,6 +42,7 @@ def test_roundtrip_bytes(all_types):
     msg = all_types.TestAllTypes.from_bytes(message_bytes)
     test_regression.check_all_types(msg)
 
+@pytest.mark.skipif(sys.version_info.major < 3, reason="mmap doesn't implement the buffer interface under python 2.")
 def test_roundtrip_bytes_mmap(all_types):
     msg = all_types.TestAllTypes.new_message()
     test_regression.init_all_types(msg)