GH-46611: [Python][C++] Allow building float16 arrays without numpy

[raulcd]

Rationale for this change

When we added Float16 we did not update pyarrow to be able to convert from Python objects to Arrow. Float16 required numpy and it crashed if numpy was not present.

What changes are included in this PR?

Allow to not require numpy to generate float16 scalars and arrays on pyarrow and do not fail if numpy is not present.

Are these changes tested?

Yes, new tests have been added

Are there any user-facing changes?

No changes for old functionality. Users will be allowed to use float16 without requiring to use np.float16 and directly from Python objects

• GitHub Issue: [Python] Float16 conversion crashes if NumPy is not installed #46611

[raulcd]

@WillAyd @pitrou what would be your expectation here:

        arr = np.array([1.5, np.nan], dtype=np.float16)
        a = pa.array(arr, type=pa.float16())
        x, y = a.to_pylist()

isinstance(x, float) or isinstance(x, np.float16)?
Currently we convert to np.float16 but in my opinion we should return a Python Float otherwise we unnecessarily require numpy.

[pitrou]

I agree a Python float should be returned. There is no reason for float16 to be different from float32 in that regard.

[pitrou]

Note this could also be arrow::util::Float16, if that's easy to do.

[raulcd]

This is requiring quite a lot of changes around pyarrow/src/arrow/python/python_to_arrow.cc, pyarrow/src/arrow/python/arrow_to_pandas.cc and haven't been able to make it work yet. I would prefer to explore updating it on a different issue