Suppress warning in test_pape_draft5.py with assertWarns.
Skip MySQL and PostgreSQL tests with the proper SkipTest exception
Work around DeprecationWarning about request.namespace in server.py
Remove overly ceremonious ElementTree import.
Cleaned up imports in xrds.py
Shorter implementation of namespaced tags lookup in xrds.py
Inline XRD elements lookup.

- Unbreak the actual lookup code to not rely on side effects
  when the direct one is in plain view.
- Inline the lookup function in iterServices.
- Write a test for finding the last XRD element, as per spec.
Remove overly complicated priority sort implementation from xrds.py. Replaced with a one-liner.