Fix mock assertion in Livy test

[topherinternational]

Looks like #41658 introduced a typo in mock assertion code, not_called() instead of assert_not_called().

This passed in Python 3.8-3.11, but Mock in 3.12 threw an error which killed 3.12 tests e.g. in #41555:

FAILED tests/providers/apache/livy/hooks/test_livy.py::TestLivyDbHook::test_post_batch_calls_get_conn_if_no_batch_id - AttributeError: 'not_called' is not a valid assertion. Use a spec for the mock if 'not_called' is meant to be an attribute.

(I guess unittest.mock in 3.12 auto-detects when you intended to assert.)

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[potiuk]

Thanks - yeah - this is common problem for Python 3.12. Possibly we should add some static check for that.

[potiuk]

The problem is that I have not seen any ready-to-use ruff or other rule for that, only Python 3.12 will start to raise those exceptions:

FAILED tests/providers/apache/livy/hooks/test_livy.py::TestLivyDbHook::test_post_batch_calls_get_conn_if_no_batch_id - AttributeError: 
'not_called' is not a valid assertion. Use a spec for the mock if 'not_called' is meant to be an attribute.

Not sure if we can automate it for Python 3.8 (and soon Python 3.9) automatically ?

[potiuk]

I created an issue in ruff repo: astral-sh/ruff#13252