Merge changes from PyXML:

Merge changes from PyXML:
[1.15]
Added understanding of the feature_validation, feature_external_pes,
and feature_string_interning features.
Added support for the feature_external_ges feature.
Added support for the property_xml_string property.
[1.16]
Made it recognize the namespace prefixes feature.
[1.17]
removed erroneous first line
[1.19]
Support optional string interning in pyexpat.
[1.21]
Restore compatibility with versions of Python that did not support weak
references.  These do not get the cyclic reference fix, but they will
continue to work as they did before.
[1.22]
Activate entity processing unless standalone.