Move pip packages to a requirement.txt file & fix exact versions ('==')

[mathieuboudreau]

This should make the notebook more reproducible in the long term.

Currently they're stored inside the Dockerfile, and only download the latest versions, thus we risk losing track of the current combination of pip packages that succesfully run the jupyter notebook(s) when new updates are made to them.

The versions of all the pip packages installed in the Binder environment can be determined inside a Jupyter Notebook session running on a MyBinder session, and running the command !pip freeze.