TST: fix test_functional tests failing because of use of pytest.approx

[NickleDave]

https://github.com/NickleDave/vak/runs/5525156504?check_suite_focus=true

=================================== FAILURES ===================================
_________________________________ test_onehot __________________________________

    def test_onehot():
        num_classes = 4
        labels = torch.zeros(2, 2, 1, dtype=torch.int64)
        labels[0, 0, 0] = 0
        labels[0, 1, 0] = 1
        labels[1, 0, 0] = 2
        labels[1, 1, 0] = 3
    
        # convert labels to one hot tensor
        one_hot = F.one_hot(labels, num_classes)
    
>       assert pytest.approx(one_hot[0, labels[0, 0, 0], 0, 0].item(), 1.0)
E       AssertionError: approx() is not supported in a boolean context.
E       Did you mean: `assert a == approx(b)`?

tests/test_nn/test_functional.py:20: AssertionError

[NickleDave]

Apparently because of this change:
pytest-dev/pytest#9061

This is what I get for not pinning my dev/test requirements