Refactor roles to use requirements files

[dometto]

Some roles now define PyPi dependencies to be installed in a rol variable/default. A downside of this is that we cannot keep dependencies up to date using Dependabot. So instead, we are always installing the latest version. This means we'll find out when a breaking change has occurred when a component fails to execute properly. A different patter could be:

• Define the dependencies for a role in a requirements.txt
• Make sure the dependencies are pinned to a certain major version
• Add the requirements file to Depandabot

This way, when the dependency has received a major update, Dependabot will open a PR and CI will run automatically, allowing us to detect breaking changes.

Roles for which this is useful

• JupyterHub
• iBridges
• Custom Packages (repo2kernel)
• ...