Use indexmap to preserve insertion order of python arrays

[kylebarron]

Originally this test failed:

def test_table_from_pydict():
    mapping = {"a": pa.array([1, 2, 3, 4]), "b": pa.array(["a", "b", "c", "d"])}
    arro3_table = Table.from_pydict(mapping)
    pa_table = pa.Table.from_pydict(mapping)
    assert pa.table(arro3_table) == pa_table

because the rust table was inferred as having column b first, and column a second. Since Python dicts are ordered, we should use an IndexMap wherever order matters (at least in from_pydict) to ensure we get the same behavior as pyarrow.