Use `unittest`'s methods for the tests

[qgallouedec]

In test, drop lines lines like

assert x in y

in favour of

self.assertIn(x, y)

[ccs96307]

Hi, I’d like to help with this issue if it's still open! I’m happy to contribute and will do my best to handle it smoothly. Thanks!

[qgallouedec]

Thanks @ccs96307! Feel free to open a PR then :)