relax dependence on canvasapi

[lsloan]

In case canvasapi changes or the application needs to use a new data source (UDP may offer a new "Canvas Data 2" data source late in 2023), make the application less dependent on that module.

Originally posted by @ssciolla in #11 (comment):

Just thought I'd sound a mild cautionary note about how tightly you are coupling this to canvasapi. If they change their implementation, or we need to move to another library, this could become difficult to migrate. It's not a huge or immediate issue, but I could see an advantage to defining our own classes and maintaining crosswalks or adapters.