Issues with legacy versions of packages

[lisi-w]

On the github.io site (https://esgf.github.io/esgf-prepare/index.html) the packages specified under the install tab as necessary do not include versions, for instance esgfetchini requires treelib version 1.4.0 specifically when installing treelib results in the installation of treelib 1.6.1 by default. Creating a venv with conda specifically to run python2 does seem to solve this issue but I think it would be helpful to many users to support python3 and current versions of packages.