Use unittest instead of nose

[arthurzam]

Summary

nose is considered deprecated, and has various issues. After some reviewing of nose usage in this library, I noticed that it is very easy to use builtin unittest functionality for it (to use assertRaises and assertRaisesRegex instead)

Also, mock is unnecessary, as you already use unittest.mock everywhere.

I have updated the docs and scripts to use python3 -m unittest discover instead of nosetests.

Checklist

• [x] Added changelog entry
• [x] Ran unit tests (nosetests tests/unit)
• [x] I understand that unless this is a Draft PR or has a DO NOT MERGE label, this PR is considered to be in a deploy ready state and can be deployed if merged to main

[saralvasquez]

Thanks for the PR! We will take a look and get back to you with any feedback/questions

[crookedneighbor]

We've dedicated some time in a few weeks to look over this and get it merged in. Thanks so much for noticing this and making the changes.

[crookedneighbor]

Thanks so much for this work @arthurzam !