&= and -= operator between Set types do not work correctly. #3992

def test_inplace_on_self(self):
    t = self.s.copy()
    t |= t
    self.assertEqual(t, self.s)
    t &= t
    self.assertEqual(t, self.s).      -> FAIL
    t -= t
    self.assertEqual(t, self.thetype()).     -> FAIL
    t = self.s.copy()
    t ^= t
    self.assertEqual(t, self.thetype())

>>>>> s = set({'a', 'b', 'c'})
>>>>> s &= s
>>>>> s
set()

>>> s = set({'a', 'b', 'c'})
>>> s &= s
>>> s
{'b', 'c', 'a'}

>>>>> s = set({'a', 'b', 'c'})
>>>>> s -= s
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: set changed size during iteration

#[pymethod(magic)]
fn iand(zelf: PyRef<Self>, set: AnySet, vm: &VirtualMachine) -> PyResult<PyRef<Self>> {
    zelf.inner
        .intersection_update(std::iter::once(set.into_iterable(vm)?), vm)?;
    Ok(zelf)
}

#[pymethod(magic)]
fn isub(zelf: PyRef<Self>, set: AnySet, vm: &VirtualMachine) -> PyResult<PyRef<Self>> {
    zelf.inner
        .difference_update(set.into_iterable_iter(vm)?, vm)?;
    Ok(zelf)
}