Previous logic assumes that `sn.type` exists before testing
if `sn` is a TerminalNode.  This PR wraps the existing logic
with an `if isinstance(sn, TerminalNode)` condition.

Also note that the `and sn.ns != self.value[1]` condition has
been removed.  Otherwise converted identityrefs produce an
invalid XML document.  FWIW, I was using the Jukebox example
from RFC 8040.  This is the identityref that wouldn't convert
otherwise:

    "genre" : "example-jukebox:alternative"

K.

Make computation more efficient by generating the NodeSet based on a
single dict comprehension instead of repeatedly calling the union
method.

Signed-off-by: Jonas Mårtensson <jonas.martensson@ri.se>

Computation is significantly faster if the path is stored as an object
attribute the first time it is generated and just returned from this
attribute for subsequent calls.

See https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

The documentation is now generated automatically for each tag
via GitLab CI.

Install importlib-metadata in CI (missing dependency for pip-tools: https://github.com/jazzband/pip-tools/issues/1352)