This conveniently pointed out potential issues in the LV2 documentation, but
makes the tool very noisy to use on anything else.  Referring to some external
thing really shouldn't be treated like a warning in linked data.  Preventing
mistakes (where full URIs are in the output instead of the expected prefixed
names) will have to be done in some other, better way (probably based on an
explicitly given set of prefixes that should be documented).

The RDFa in specification pages doesn't actually use prefixed names for the
documented resources.  So, simplify things by trimming the namespaces down to a
consistent set that is actually used in the LV2 universe.

Switches to safer and more modern idioms, and addresses many issues raised by
flake8 and pylint.

Python 2 hit EOL long ago, and some systems don't have a "python" at all.