docs: Keep up with the location of NoUri

Sphinx 2.1 moved sphinx.environment.NoUri into sphinx.errors; that produced
this warning in the docs build:

  /usr/lib/python3.7/site-packages/sphinx/registry.py:473:
    RemovedInSphinx30Warning: sphinx.environment.NoUri is deprecated.

Grab NoUri from the right place and make the warning go away.  That symbol
was only added to sphinx.errors in 2.1, so we must still import it from the
old location when running in older versions.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>