Missing slots for AmdnmentLineMixin in product kit modules

We noticed it when running the test suite of a custom module that depends on both product_kit and purchase_amendment. This is not detected by the test suite as the modules are not added as extras on test suite.

After adding them I get the following traceback:

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FAIL: test_pool_slots (test_module.ProductKitTestCase.test_pool_slots) (type='model', name='purchase.amendment.line')
Test pool object has __slots__
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Traceback (most recent call last):
  File "/home/pokoli/projectes/tryton/trytond/trytond/tests/test_tryton.py", line 858, in test_pool_slots
    self.assertTrue(hasattr(kls, '__slots__'),
AssertionError: False is not true : The <class 'trytond.modules.product_kit.common.AmendmentLineMixin'> of model 'purchase.amendment.line' has no __slots__

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FAIL: test_pool_slots (test_module.ProductKitTestCase.test_pool_slots) (type='model', name='sale.amendment.line')
Test pool object has __slots__
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Traceback (most recent call last):
  File "/home/pokoli/projectes/tryton/trytond/trytond/tests/test_tryton.py", line 858, in test_pool_slots
    self.assertTrue(hasattr(kls, '__slots__'),
AssertionError: False is not true : The <class 'trytond.modules.product_kit.common.AmendmentLineMixin'> of model 'sale.amendment.line' has no __slots__