Switch to pytest

[cbrnr]

nose is not actively maintained anymore, so I suggest to switch to pytest.

[mbillingr]

Is pytest a drop-in replacement for nose or are there any gotchas?

[cbrnr]

I think our tests should work as is. pytest seems to be more flexible though, so we could refactor our tests at a later point if we want to.

[mbillingr]

In that case, +1 from me :)