Tests Refresh

[mire3212]

• Tweaking a value for the client.py URL, as I think it's causing 404 errors with the new API URL Endpoint.
• Re-adding the missing warn import statement (not sure if we still need the one line that uses warn(), but it's there none-the-less).
• Aligning headers value during authentication with what is shown in the docs.
• Updating test files to use the new API endpoint.
• Adding config for pytest to help locate the project code.
• Removing some unused tests.
• Updating some tests that are referencing old or wrong error codes.

On my local system pytest now passes all tests (though, it might be good to confirm they're all still good).