As discussed in #517.

Implement setup_*/teardown_* using fixtures injected during collection.

During collection, TestCase subclasses with a setup_ and/or teardown_ methods would have this fixture injected in the class:

@pytest.fixture(autouse=True)
def _xunit_setup_teardown_function(self):
    if hasattr(self, 'setup_method'):
        self.setup_method(self)
    yield
    if hasattr(self, 'teardown_method'):
        self.teardown_method(self)

If neither method is present, then this fixture is not injected at all.

The same would be done for setUpClass and setUpModule.

This would solve the problems mentioned in #517 quite nicely.

As discussed, this change might introduce subtle errors so this might be better introduced with an option to revert to the old implementation.

This should no longer be a concern, because we will only handle pytest's methods this way.

EDIT: updated regarding #3094 (comment)