Add pooch to environment.yml file

[mtb-za]

At least one notebook in the current master branch uses pooch, so we need to make sure that it is installed with everything else. We expect that more will use it, moving forwards.

[kwinkunks]

If we end up using pooch a lot, then it belongs in environment.yaml, but otherwise the thing to do is to place it in the YAML control file for any course that uses the notebook(s) that require it. It's only libraries that all (or at least most) of the classes need that go in environment.yaml.

In other words, it's not the case that building an env with environment.yaml will enable a person to run all the notebooks in prod. Right now, there's no way to do that because there's no way to survey the notebooks either en masse or (say) from a list in a course YAML file. That would be a cool feature and is related to Issue #3 in kosu.