try to decode some malicious xml on startup; if this succeeds,
then force-disable all xml-based features (primarily WebDAV)
this is paranoid future-proofing against unanticipated changes
in future versions of python, specifically if the importlib or
xml.etree.ET behavior changes in a way that somehow reenables
entity expansion, which (still hypothetically) would probably
be caused by failing to unload the `_elementtree` c-module
no past or present python versions are affected by this change
