Test failure under PyPy 3.9

[tseaver]

Found this while adding GHA actions to PR #307.

$ tox - pypy39
...
=================================== FAILURES ===================================
____________ TestEvolutionManager.test_get_finished_steps_by_value _____________

self = <substanced.evolution.tests.test_evolution.TestEvolutionManager testMethod=test_get_finished_steps_by_value>

    def test_get_finished_steps_by_value(self):
        root = DummyRoot()
        inst = self._makeOne(root, None, None)
        inst.add_finished_step('abc')
        inst.add_finished_step('def')
>       steps = inst.get_finished_steps_by_value()

inst       = <substanced.evolution.EvolutionManager object at 0x0000000004483ef8>
root       = <substanced.evolution.tests.test_evolution.DummyRoot object at 0x0000000004483f30>
self       = <substanced.evolution.tests.test_evolution.TestEvolutionManager testMethod=test_get_finished_steps_by_value>

substanced/evolution/tests/test_evolution.py:37: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <substanced.evolution.EvolutionManager object at 0x0000000004483ef8>

    def get_finished_steps_by_value(self):
>       return reversed(self.get_finished_steps().byValue(0))
E       TypeError: 'reversesequenceiterator' has no length

self       = <substanced.evolution.EvolutionManager object at 0x0000000004483ef8>

substanced/evolution/__init__.py:48: TypeError

For now, I'm just disabling the CI for `pypy39' in that PR.