ubiquity fails it's autopkg tests with Python 3.12

[Matthias Klose]

ubiquity fails it's autopkg tests with Python 3.12:

https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/amd64/u/ubiquity/20240126_143030_c2809@/log.gz

727s
727s ======================================================================
727s ERROR: test_add_ap (test_gtkwidgets.NetworkStoreTests.test_add_ap)
727s ----------------------------------------------------------------------
727s Traceback (most recent call last):
727s File "/tmp/autopkgtest.lgl8DR/build.107/src/tests/test_gtkwidgets.py", line 121, in test_add_ap
727s self.assert_(ap_it)
727s ^^^^^^^^^^^^
727s AttributeError: 'NetworkStoreTests' object has no attribute 'assert_'. Did you mean: 'assertIn'?
727s
727s ======================================================================
727s ERROR: test_has_ap (test_gtkwidgets.NetworkStoreTests.test_has_ap)
727s ----------------------------------------------------------------------
727s Traceback (most recent call last):
727s File "/tmp/autopkgtest.lgl8DR/build.107/src/tests/test_gtkwidgets.py", line 136, in test_has_ap
727s self.assert_(self.model.has_ap('/foo', 'Orange'))
727s ^^^^^^^^^^^^
727s AttributeError: 'NetworkStoreTests' object has no attribute 'assert_'. Did you mean: 'assertIn'?
727s
727s ======================================================================
727s ERROR: test_has_device (test_gtkwidgets.NetworkStoreTests.test_has_device)
727s ----------------------------------------------------------------------
727s Traceback (most recent call last):
727s File "/tmp/autopkgtest.lgl8DR/build.107/src/tests/test_gtkwidgets.py", line 104, in test_has_device
727s self.assert_(self.model.has_device('/foo'))
727s ^^^^^^^^^^^^
727s AttributeError: 'NetworkStoreTests' object has no attribute 'assert_'. Did you mean: 'assertIn'?
727s
727s ======================================================================
727s FAIL: test_hostname_check_exists (test_usersetup.UserSetupTests.test_hostname_check_exists)
727s ----------------------------------------------------------------------
727s Traceback (most recent call last):
727s File "/tmp/autopkgtest.lgl8DR/build.107/src/tests/test_usersetup.py", line 51, in test_hostname_check_exists
727s self.gtk.hostname_error.assert_called_with(error_msg)
727s File "/usr/lib/python3/dist-packages/mock/mock.py", line 980, in assert_called_with
727s raise AssertionError(_error_message()) from cause
727s AssertionError: expected call not found.
727s Expected: mock('That name already exists on the network.')
727s Actual: mock(None)
727s
727s ----------------------------------------------------------------------
727s Ran 92 tests in 13.919s
727s
727s FAILED (failures=1, errors=4, skipped=1)
728s autopkgtest [14:30:18]: test unit: -----------------------]