Fix numpy iterable import in _causal_analysis.py for numpy>2.0

[imatiach-msft]

With new numpy 2.0 update the numpy.lib.function_base namespace has become private.
If you try to import it you get the error:

    raise AttributeError(
AttributeError: numpy.lib.function_base is now private. If you are using a public function, it should be available in the main numpy namespace, otherwise check the NumPy 2.0 migration guide.

In econml this throws the exception:

/usr/share/miniconda/envs/test/lib/python3.9/site-packages/responsibleai/managers/causal_manager.py:10: in <module>
    from econml.solutions.causal_analysis import CausalAnalysis
/usr/share/miniconda/envs/test/lib/python3.9/site-packages/econml/solutions/causal_analysis/__init__.py:4: in <module>
    from ._causal_analysis import CausalAnalysis
/usr/share/miniconda/envs/test/lib/python3.9/site-packages/econml/solutions/causal_analysis/_causal_analysis.py:13: in <module>
    from numpy.lib.function_base import iterable
E   ModuleNotFoundError: No module named 'numpy.lib.function_base'

The fix is to import the function from numpy directly, which is also backwards compatible.

[kbattocchi]

Looks good, thanks!

[imatiach-msft]

@kbattocchi it looks like one of the checks to build documentation is failing, preventing a merge:
https://github.com/py-why/EconML/actions/runs/12697247100/job/35393115771?pr=945
can that build be re-tried? I don't have permissions to re-try it. Otherwise, maybe there is some issue with the documentation build that needs to be fixed.

[kbattocchi]

@imatiach-msft The documentation problem is unrelated to your change; I'm working on a fix and will merge it into your PR once it's in, and then will merge your PR as well, you don't need to do anything further.