ValueError break with lxml 3.5.0

[daveschaefer]

In lxml 3.5.0 the searching behaviour was changed to explicitly reject None type namespace mappings. From the changelog: "Empty prefixes are explicitly rejected when a namespace mapping is used with ElementPath to avoid hiding bugs in user code"[1]

This breaks checkloc because it's currently using the dynamically generated namespace mappings generated by the root; this causes the namespace dictionary to contain a None entry.

Reported under danwent/Perspectives#166

[1] http://lxml.de/3.5/changes-3.5.0.html

[daveschaefer]

The fix for this is simple - we can just explicitly use only the namespace(s) we need.

[daveschaefer]

Fixed!