Support strict namespace
OOXML files are a mess. The specification was ratified after Microsoft released Office 2007 and contain subtle differences. Complicating matters was the decision to use a different namespace once the specification had been ratified. We implement the specification wherever we can, because it is at least documented, but follow the general practice of using the 2006 transitional namespaces. As supporting different namespaces for the same things isn't trivial in Python we can't read or write using the "strict" namespaces.