Fix python bindings with versions older than 2.7

Need fixing on the Capsule usage, the lack of PyBytes,
lack of io module and the way to access exception details.

python/tests/reader8.py

@@ -3,7 +3,6 @@
 # this tests the entities substitutions with the XmlTextReader interface
 #
 import sys
-import io
 import libxml2
 
 # Memory debug specific