Investigate why the `matching` notebooks cannot be validated

[drvinceknight]

In 1babbc5 (in this PR: #147) I had to skip the validation of two cells. I have no idea why, note that they had to be skipped and it is not that the output was different to what was expected.

[drvinceknight]

Looking at the rendered page https://vknight.org/pfm/further-information/04-pip-installing/tutorial/main.html the following warnings appear:

/opt/hostedtoolcache/Python/3.8.6/x64/lib/python3.8/site-packages/ipykernel/ipkernel.py:287: DeprecationWarning: `should_run_async` will not call `transform_cell` automatically in the future. Please pass the result to `transformed_cell` argument and any exception that happen during thetransform in `preprocessing_exc_tuple` in IPython 7.17 and above.
  and should_run_async(code)

and

/opt/hostedtoolcache/Python/3.8.6/x64/lib/python3.8/site-packages/ipykernel/ipkernel.py:287: DeprecationWarning: `should_run_async` will not call `transform_cell` automatically in the future. Please pass the result to `transformed_cell` argument and any exception that happen during thetransform in `preprocessing_exc_tuple` in IPython 7.17 and above.
  and should_run_async(code)

Screenshot: