Fix compatibility with rdflib 6.0.0

[osma]

Fixes #86

In rdflib 6.0.0, the RDF and RDFS namespaces were changed from ClosedNamespace to DeclaredNamespace (see RDFLib/rdflib#1074). This also dropped the .uri attribute that Skosify was relying on.

This PR switches to another way to get the URI of these namespaces (using str()). It works with both rdflib 5.0.0 and 6.0.0.

[codecov-commenter]

Codecov Report

Merging #87 (0eaa1c7) into master (0bcb8d7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   59.24%   59.24%           
=======================================
  Files          11       11           
  Lines         930      930           
=======================================
  Hits          551      551           
  Misses        379      379           

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