Spanner: Building the docs fails

[plamut]

Building the docs emits a warning, which is interpreted as error:

$ nox -f noxfile.py -s docs

...
Traceback (most recent call last):
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app.build(args.force_all, filenames)
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/application.py", line 353, in build
    self.builder.build_update()
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 299, in build_update
    len(to_build))
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 361, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 535, in write
    self._write_serial(sorted(docnames))
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 545, in _write_serial
    self.write_doc(docname, doctree)
  File "/usr/local/lib/python3.7/contextlib.py", line 119, in __exit__
    next(self.gen)
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 219, in pending_warnings
    memhandler.flushTo(logger)
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 184, in flushTo
    logger.handle(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 1478, in handle
    self.callHandlers(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 1540, in callHandlers
    hdlr.handle(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 850, in handle
    rv = self.filter(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 712, in filter
    result = f.filter(record)
  File "/home/peter/workspace/google-cloud-python/spanner/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 404, in filter
    raise SphinxWarning(location + ":" + message)
sphinx.errors.SphinxWarning: /home/peter/workspace/google-cloud-python/spanner/docs/instance-api.rst:11:Could not lex literal_block as "python". Highlighting skipped.