Increase Test Coverage

[allerter]

I tested the coverage of the unit tests using coveragepy, and the coverage stands at about %75. The HTML report is available here.
After releasing LyricsGenius 3.0.0, there were some issues that we could've easily avoided if we had more tests (#181, #182, #183, #184) . Achieving a coverage of %95 or more will help prevent these issues going forward. But before doing so, we should migrate the tests to pytest to make it easier to add more tests (#205).

To reproduce the report:

git clone https://github.com/allerter/LyricsGenius.git
cd LyricsGenius
git checkout test-coverage
pip install -e .[dev]
tox -e coverage
coverage html