beautifulsoup:
  Format text/html and image/svg+xml outputs with beautiful-soup.

  See: https://beautiful-soup-4.readthedocs.io.

blacken_code:
  Format python source code with black (see https://black.readthedocs.io).

coalesce_streams:
  Merge all stream outputs with shared names into single streams.

  This ensure deterministic outputs.

  Adapted from:
  https://github.com/computationalmodelling/nbval/blob/master/nbval/plugin.py.
