Use pytest instead of nose

[jnns]

nosetests doesn't support Python 3.10 and is also unmaintained. The comment in tests.yml suggested that pytest should be used instead. This pull request replaces nose with pytest.

[millerdev]

The comment in tests.yml suggested that pytest should be used instead.

Indeed, it suggested that maybe that was an option. However, I really dislike pytest's magical/unpythonic fixtures, and I was hoping to find a way to avoid using them before making the switch. I'll think about this.

[jnns]

Do you know Ward already? I have no experience with it whatsoever but I read that it handles fixture resolution differently.

[millerdev]

Thanks for the suggestion, I'll take a look.

[a-detiste]

ping ... the great nose removal is still ongoing